[LeetCode] Find the Duplicate Number

Find the Duplicate Number

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

Note:

1. You must not modify the array (assume the array is read only).
2. You must use only constant extra space.
3. Your runtime complexity should be less than O(n2).

二分查找，复杂度 O(nlogn)。

class Solution {
public:
    int findDuplicate(vector<int>& nums) {
        int l = 0, r = nums.size() - 1, m;
        while (l <= r) {
            m = l + ((r - l) >> 1);
            int cnt = 0;
            for (auto a : nums) if (a <= m) ++cnt;
            if (cnt <= m) l = m + 1;
            else r = m - 1;
        }
        return l;
    }
};