力扣704 - Binary Search 二分查找

题目描述

Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists, then return its index. Otherwise, return -1.

Example 1:

Input: nums = [-1,0,3,5,9,12], target = 9
Output: 4
Explanation: 9 exists in nums and its index is 4

Example 2:

Input: nums = [-1,0,3,5,9,12], target = 2
Output: -1
Explanation: 2 does not exist in nums so return -1

典型的二分查找题, 需要注意的是区间问题和边界值

代码

1. 最简单但是效率差的python解法：

class Solution:
    def search(self, nums: List[int], target: int) -> int:
        if target in nums:
            return nums.index(target)
        else:
            return -1

2. 因为最近在学java，就用java解题了。

正确的二分查找算法：

class Solution {
    public int search(int[] nums, int target) {
        int left = 0;
        int right = nums.length -1;
        while (left <= right){
            int mid = (left + right)/2;
            // found
            if (nums[mid] == target){
                return mid;
            }
            //中间比target小，往右搜
            else if (nums[mid] < target)
                left = mid + 1; //注意区间问题
            else if (nums[mid] >= target)
                right = mid -1 ;     
        }
        //没找到
        return -1;
    }
}