Find the Duplicate Number

题目
Given an array nums containing n + 1 integers where each integer is between 1 and n (inclusive), prove that at least one duplicate number must exist. Assume that there is only one duplicate number, find the duplicate one.

答案

class Solution {
    public int findDuplicate(int[] nums) {
        int l = 1, r = nums.length - 1;
        while(l < r) {
            int m = (l + r) / 2;
            int cnt = 0;
            for(int num : nums) {
                if(num <= m) cnt++;
            }
            if(cnt > m) {
                // Go left
                r = m;
            }
            else {
                l = m + 1;
            }
        }
        return l;
    }
}