LeetCode-704.Binary Search

Given a sorted (in ascending order) integer array nums of n elements and a targetvalue, 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

Note:

1. You may assume that all elements in nums are unique.
2. n will be in the range [1, 10000].
3. The value of each element in nums will be in the range [-9999, 9999].

二分查找 时间复杂度O(lgn)

    public int search(int[] nums, int target) {
        if(null==nums||0==nums.length){
            return -1;
        }
        int left = 0;
        int right = nums.length-1;
        while(left<= right){
            int mid = (left+right)/2;
            if(nums[mid] == target){
                return mid;
            }
            else if(nums[mid] >target){
                right = mid-1;
            }
            else{
                left = mid+1;
            }
        }
        return -1;
    }

 递归

    public int search(int[] nums, int target) {
        if(null==nums||0==nums.length){
            return -1;
        }
        
        return binaryS(nums,0,nums.length-1,target);
    }
    private int binaryS(int[] nums,int left,int right,int target){
        if(left>right){
            return -1;
        }
        int mid = (left+right)/2;
        if(nums[mid]==target){
            return mid;
        }
        if(nums[mid]<target){
            return binaryS(nums,mid+1,right,target);
        }
        return binaryS(nums,left,mid-1,target);
    }