思路：看官方解答

class Solution {
public:int findDuplicate(vector<int>& nums) {int n=nums.size();int left=1;int right=n-1;int ans=-1;while(left<=right){int mid=(left+right)/2;int count=0;for(int j=0;j<n;j++){if(nums[j]<=mid){count++;}}if(count<=mid){left=mid+1;}else{right=mid-1;ans=mid;}}return ans;}
};