leetcode 寻找重复数

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        int n = nums.size();
        int l = 1, r = n-1, ans = -1;
        while(l <= r) {
            int mid = (l+r)/2;
            int cnt = 0;
            for(int i=0; i<n; i++) {
                cnt += nums[i] <= mid;
            }
            if(cnt <= mid)
                l = mid + 1;
            else {
                r = mid - 1;
                ans = mid;
            }
        }
        return ans;
    }
};