Python二分查找详解

先来看个实例

    #!/usr/bin/env python 
    import sys  

    def search2(a,m): 
      low = 0  
      high = len(a) - 1  
      while(low <= high): 
        mid = (low + high)/2 
        midval = a[mid] 

        if midval < m: 
          low = mid + 1  
        elif midval > m: 
          high = mid - 1  
        else: 
          print mid  
          return mid  
      print -1 
      return -1 

    if __name__ == "__main__": 
      a = [int(i) for i in list(sys.argv[1])] 
      m = int(sys.argv[2]) 
      search2(a,m) 

运行:

    administrator@ubuntu:~/Python$ python test_search2.py 123456789 4

3

注:

1.''：由于python的类成员都是公有、公开的被存取public，缺少像正统面向对象语言的私有private属性。
于是就用来将就一下，模拟私有属性。这些属性往往是内部使用，通常情况下不用改写。也不用读取。
加上2个下划线的目的，一是不和普通公有属性重名冲突，二是不让对象的使用者(非开发者)随意使用。
2.name == "main"表示程序脚本是直接被执行的.
如果不等于表示脚本是被其他程序用import引入的.则其name__属性被设为模块名

Python采用二分查找找出数字的下标

要考虑有重复数字的情况

    class Solution(object):
      def searchRange(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        def binary_search(start,end,value):
          while end>=start:
            mid = (start+end)//2
            print(mid)
            if nums[mid]>target:
              end = mid-1
            elif nums[mid]<target:
              start = mid+1
            else: 
              if value==-1:
                if mid-1>=start and nums[mid+value] == target:
                  end = mid+value
                else:
                  return mid
              else:
                if mid+1<=end and nums[mid+value] == target:
                  start = mid+value
                else:
                  return mid

          return -1
        a=binary_search(0,len(nums)-1,-1)
        b=binary_search(0,len(nums)-1,1)
        return [a,b]
    a = Solution()
    l = [2,2]
    print(a.searchRange(l,2))

二分算法的定义不在多说了，百度一下就知道（支持国产大笑）

    import sys 
    source = [1,2,3,4,5,6,7,8,9,10] #must be in order 
    des = int(sys.argv[1]) 
    low = 0 
    high = len(source) - 1 
    targetIndex = -1 
    print "des=",des 
    while low <= high: 
      middle = (low + high)/2 
      if des == source[middle]: 
        targetIndex = middle 
        break 
      elif des < source[middle]: 
        high = middle -1 
        print "middle element[index=",middle,",value=",source[middle],"] is bigger than des, continue search from[",low,"to",high,"]" 
      else: 
        low = middle + 1 
        print "middle element[index=",middle,",value=",source[middle],"] is smaller than des, continue search from[",low,"to",high,"]" 
    print "search complete, target element's index in source list is ",targetIndex 

最后在分享一个

    'fileName--BinarySearch.py' 

    src = [] 

    def BinarySearch(low, high, target, *src): 
      '二分查找' 
      while low <= high: 
        mid = (low + high) // 2 
        midVal = src[mid] 
        if target < midVal: 
          high = mid - 1 
        elif target > midVal: 
          low = mid + 1 
        else: 
          return mid 
        BinarySearch(low, high, target, *src) 

    print('Please input 10 number:') 
    for number in range(10): 
      src.append(int(input('Num %d:' % number))) 

    sortList = tuple(src) 

    key = int(input('Please input key:')) 
    location = BinarySearch(0, len(src) - 1, key, *sortList) 

    if location != None: 
      print('Find target at %d' % (location + 1)) 
    else: 
      print('No target!')