253 lines
5.3 KiB
Markdown
253 lines
5.3 KiB
Markdown
# 原创
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： Python数据结构（五）——排序和搜索
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# Python数据结构（五）——排序和搜索
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## 排序和搜索
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```
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15 in [3,3,2,1,4]
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```
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```
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False
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```
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```
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3 in [3,4,5,6]
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```
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```
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True
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```
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### 顺序查找
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```
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# 查找列表中的项,假设列表项无序
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def sequence_search(alist,item):
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pos = 0
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found = False
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while pos<len(alist) and not found:
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if alist[pos]==item:
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found = True
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else:
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pos += 1
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return found
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testlist = [1, 2, 32, 8, 17, 19, 42, 13, 0]
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print(sequence_search(testlist, 3))
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print(sequence_search(testlist, 13))
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```
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```
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False
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True
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```
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```
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# 查找列表中的项,假设列表项有序
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def order_sequence_search(alist,item):
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pos = 0
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found = False
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stop = False
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while pos < len(alist) and not found and not stop:
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if alist[pos] == item:
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found = True
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else:
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if alist[pos]>item:
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stop = True
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else:
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pos += 1
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return found
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testlist = [0, 1, 2, 8, 13, 17, 19, 32, 42,]
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print(order_sequence_search(testlist, 3))
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print(order_sequence_search(testlist, 13))
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```
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```
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False
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True
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```
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### 二分法查找
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```
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def binary_search(alist,item):
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first = 0
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last = len(alist)-1
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found = False
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while first<=last and not found:
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mid = (first+last)/2
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if alist[mid]==item:
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found = True
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elif alist[mid]>item:
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last = mid - 1
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else:
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first = mid + 1
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return found
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testlist = [0, 1, 2, 8, 13, 17, 19, 32, 42,]
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print(binary_search(testlist, 3))
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print(binary_search(testlist, 13))
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```
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```
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False
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True
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```
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```
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# 递归实现
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def bianary_search(alist,item):
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if len(alist)==0:
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return False
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else:
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mid = len(alist)//2
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if alist[mid]==item:
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return True
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else:
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if item<alist[mid]:
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return bianary_search(alist[:mid],item)
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else:
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return bianary_search(alist[mid+1:],item)
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testlist = [0, 1, 2, 8, 13, 17, 19, 32, 42,]
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print(binary_search(testlist, 3))
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print(binary_search(testlist, 13))
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```
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```
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False
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True
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```
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### Hash查找
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哈希表 是以一种容易找到它们的方式存储的项的集合。哈希表的每个位置，通常称为一个槽，可以容纳一个项，并且由从 0 开始的整数值命名。例如，我们有一个名为 0 的槽，名为 1 的槽，名为 2 的槽，以上。最初，哈希表不包含项，因此每个槽都为空。我们可以通过使用列表来实现一个哈希表，每个元素初始化为None 。Figure 4 展示了大小 m = 11 的哈希表。换句话说，在表中有 m 个槽，命名为 0 到 10。 <br/> <img alt="" src="https://raw.githubusercontent.com/ds19991999/githubimg/master/picgo/20180730133625.png" title=""/>
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具体介绍见：[Hash查找](https://github.com/facert/python-data-structure-cn/tree/master/5.%E6%8E%92%E5%BA%8F%E5%92%8C%E6%90%9C%E7%B4%A2/5.5.Hash%E6%9F%A5%E6%89%BE)
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```
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def hash(astring, tablesize):
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sum = 0
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for pos in range(len(astring)):
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sum = sum+ord(astring[pos])
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return sum%tablesize
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```
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冲突解决： <br/> <img alt="" src="https://raw.githubusercontent.com/ds19991999/githubimg/master/picgo/20180730135953.png" title=""/>
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### 排序
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```
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# 冒泡排序
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def bubble_sort_1(alist):
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for j in range(len(alist)-1,0,-1):
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for i in range(j):
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if alist[i]>alist[i+1]:
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alist[i],alist[i+1]=alist[i+1],alist[i]
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return alist
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alist = [54,26,93,17,77,31,44,55,20]
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print bubble_sort_1(alist)
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```
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```
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[17, 20, 26, 31, 44, 54, 55, 77, 93]
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```
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```
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# 优化冒泡排序，识别有序序列,修改冒泡排序提前停止
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def bubble_sort_2(alist):
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exchange = True
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j = len(alist)-1
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while j>0 and exchange:
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exchange = False
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for i in range(j):
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if alist[i] > alist[i+1]:
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alist[i],alist[i+1]=alist[i+1],alist[i]
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exchange = True
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j -= 1
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return alist
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alist=[30,20,40,90,50,60,70,80,100,110]
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print bubble_sort_2(alist)
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```
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```
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[20, 30, 40, 50, 60, 70, 80, 90, 100, 110]
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```
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```
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# 简单选择排序
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def select_sort(alist):
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for i in range(len(alist)):
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k = i
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for j in range(k,len(alist)):
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if alist[k]>alist[j]:
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k = j
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alist[i],alist[k]=alist[k],alist[i]
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return alist
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alist = [54,26,93,17,77,31,44,55,20]
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print select_sort(alist)
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```
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```
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[17, 20, 26, 31, 44, 54, 55, 77, 93]
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```
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```
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# 插入排序
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def insert_sort(alist):
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for i in range(0,len(alist)):
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for j in range(i+1,len(alist)):
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if alist[i]>alist[j]:
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tmp = alist[j]
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alist.pop(j)
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alist.insert(i,tmp)
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return alist
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alist = [54,26,93,17,77,31,44,55,20]
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print insert_sort(alist)
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```
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```
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[17, 20, 26, 31, 44, 54, 55, 77, 93]
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```
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```
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# 插入排序2
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def insert_sort_2(A):
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length = len(A)
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if length < 2:
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return A
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for i in range(1,length-1):
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key = A[i]
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j = i-1
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while j>=0 and A[j]>key:
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A[j+1]=A[j]
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j -= 1
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A[j+1] = key
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return A
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alist = [54,26,93,17,77,31,44,55,20]
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print insert_sort_2(alist)
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```
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```
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[17, 26, 31, 44, 54, 55, 77, 93, 20]
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```
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更多排序算法见博客：[Python排序算法](https://blog.csdn.net/ds19991999/article/details/79998011)
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