二分查找（折半查找）

1、寻找一个数

  1. public int binarySearch(int[] nums, int target) {
  2.     int left = 0;
  3.     int right = nums.length - 1;
  4.     while(left <= right) {    
  5.         int mid = (right - left) / 2 + left;    //取中间数
  6.         if (nums[mid] == target) {    
  7.             return target;
  8.         }else if (nums[mid] < target) {
  9.             left = mid + 1;    
  10.         }else if (nums[mid] > target) {
  11.             right = mid - 1;    
  12.         }
  13.     }
  14.     return ...;
  15. }

2、寻找左侧边界

3、寻找右侧边界