查找算法——二分查找

1、算法介绍

（1）序列必须是有序的

（2）取序列的中间元素与查找值比较，相等返回元素下标

（3）根据序列升序或降序，中间元素小于或大于查找值的具体情况，选择中间元素右边或左边的序列重复步骤（2）（3），

未找到返回-1。

2、代码实现

2.1、golang

package main

import (
	"fmt"
)

func main() {
	slice := []int{1, 2, 3, 4, 5, 6} //升序序列
	key := 4
	index := SearchBinary(slice, key)
	if index == -1 {
		fmt.Printf("%v不存在元素%v\n", slice, key)
	} else {
		fmt.Printf("%v位于%v下标为%v的位置。\n", key, slice, index)
	}
}

//二分查找
func SearchBinary(slice []int, key int) int {
	left, right := 0, len(slice)-1
	for left <= right {
		mid := (left + right) / 2
		if slice[mid] == key {
			return mid
		}
		if slice[mid] > key {
			right = mid - 1
		}
		if slice[mid] < key {
			left = mid + 1
		}
	}
	return -1
}

2.2、python3

# 二分查找
def search_binary(arr, key):
    left, right = 0, len(arr) - 1
    while (left <= right):
        mid = (left + right) // 2
        if (arr[mid] == key):
            return mid
        if (arr[mid] > key):
            right = mid - 1
        if (arr[mid] < key):
            left = mid + 1
    return -1


# 递归二分查找
def search_binary2(arr, key, left, right):
    if (left > right):
        return -1
    mid = (left + right) // 2
    if (arr[mid] == key):
        return mid
    if (arr[mid] > key):
        return search_binary2(arr, key, left, mid - 1)
    if (arr[mid] < key):
        return search_binary2(arr, key, mid + 1, right)


if __name__ == '__main__':
    arr = [1, 2, 3, 4, 5, 6]
    key = 6
    # index = search_binary(arr, key)
    index = search_binary2(arr, key, 0, len(arr) - 1)
    if index == -1:
        print("%s不存在元素%s。" % (arr, key))
    else:
        print("%s位于%s下标为%s的位置。" % (key, arr, index))