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;

    }
};