LeetCode – Find the Duplicate Number (Java)

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.

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.

Java Solution 1 - Wrong

This solution is wrong, but the same idea is used in Solution 3.

public int findDuplicate(int[] arr) {
    for(int i=0; i<arr.length; i++){
                return arr[i];
            int t = arr[arr[i]-1];
    return -1;

Java Solution 2 - Binary Search

public int findDuplicate(int[] nums) {
    int l=1,r=nums.length-1;
        int m=(l+r)/2;
        int c=0;
        for(int i: nums){
        //if c < m, 
    return r;

Java Solution 3 - Finding Cycle

public int findDuplicate(int[] nums) {
    int slow = 0;
    int fast = 0;
        slow = nums[slow];
        fast = nums[nums[fast]];
    } while(slow != fast);
    int find = 0;
    while(find != slow){
        slow = nums[slow];
        find = nums[find];
    return find;
Category >> Algorithms  
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  • Yi-jhe Huang

    Using the math solution:
    totalSum – n*(n+1)/2
    = 4 – (2*3/2)
    = 4 – 3
    = 1

  • Ankit Shah

    okay this makes sense with the example your solution might work

  • Yi-jhe Huang

    Let n be the maximum element in the input nums array.
    Can you take one case as an example?

  • Ankit Shah

    Hi yi-jhe Huang, that will only work if the n starts at 0. here the question is n starts from 1.

  • Yi-jhe Huang

    How about using the math solution:
    totalSum – n*(n+1)/2