二分法

写法:

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

进阶:

class Solution {
    public int search(int[] nums, int target) {
        // 避免当 target 小于nums[0] nums[nums.length - 1]时多次循环运算
        if (target < nums[0] || target > nums[nums.length - 1]) {
            return -1;
        }
        int left = 0, right = nums.length - 1;
        while (left <= right) {
            int mid = left + ((right - left) >> 1);
            if (nums[mid] == target)
                return mid;
            else if (nums[mid] < target)
                left = mid + 1;
            else if (nums[mid] > target)
                right = mid - 1;
        }
        return -1;
    }
}