经典二分，查找左右边界的二分法！！！！！！

class Solution {
public:
    int search(vector<int>& nums, int target) {
        int left=0,right=nums.size()-1;
        //查找右边界,让i不断增加
        while(left<=right){
            int mid=left+(right-left)/2;
            if(nums[mid]<=target) left++;
            else right--;
        } 
        int j=left;
        if(right<0 || nums[right]!=target) return 0;
       //查找左边界,让右边界不断移动出界
       left=0,right=nums.size()-1;
       while(left<=right){
           int mid=left+(right-left)/2;
           if(nums[mid]>=target) right--;
           else left++;
       }
       int i=right;
       return j-i-1;
    }
};