LeetCode-in-All

287. Find the Duplicate Number

Medium

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?

Solution

#include <stdio.h>

int findDuplicate(int* nums, int numsSize) {
    // Create an array of size numsSize + 1, initialized to 0
    int arr[numsSize + 1];
    for (int i = 0; i <= numsSize; i++) {
        arr[i] = 0;
    }

    // Iterate over the nums array and update the arr counts
    for (int i = 0; i < numsSize; i++) {
        arr[nums[i]] += 1;
        if (arr[nums[i]] == 2) {
            return nums[i];  // Return the duplicate element
        }
    }

    return 0;  // In case no duplicate is found, though the problem assumes there is always a duplicate
}

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