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 number must exist. Assume that there is only one duplicate number, find the duplicate one.

Note:

You must not modify the array (assume the array is read only).

You must use only constant, O(1) extra space.

Your runtime complexity should be less than

O(n2)

.

There is only one duplicate number in the array, but it could be repeated more than once.

Runtime: 28ms

class Solution {
public:
int findDuplicate(vector<int>& nums) {
if(nums.empty()) return 0;

unordered_map<int, int> m;
for(int i = 0; i < nums.size(); i++){
if(m.find(nums[i]) == m.end())
m[nums[i]] = i;
else return nums[i];
}
}
};