1 Description #

source: https://leetcode.com/problems/find-the-duplicate-number/

Given an array of integers nums containing n + 1 integers where each integer is in the range [1, n] inclusive.

There is only one repeated number in nums, return this repeated number.

You must solve the problem without modifying the array nums and uses only constant extra space.

Example 1:

Input: nums = [1,3,4,2,2]
Output: 2

Example 2:

Input: nums = [3,1,3,4,2]
Output: 3

Constraints:

• 1 <= n <= 105
• nums.length == n + 1
• 1 <= nums[i] <= n
• All the integers in nums appear only once except for precisely one integer which appears two or more times.

Follow up:

• How can we prove that at least one duplicate number must exist in nums?
• Can you solve the problem in linear runtime complexity?

2 Solution #

class Solution {
public:
  int findDuplicate(vector<int>& nums){
    // Since each integer is in the range [1, n] inclusive, then just map the number to index
    // Time complexity: O(n)
    // Space complexity: O(1)
    while(nums[0] != nums[nums[0]]){
      std::swap(nums[0], nums[nums[0]]);
    }
    return nums[0];
  }
};