LeetCode 287

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.

/*************************************************************************
    > File Name: LeetCode287.c
    > Author: Juntaran
    > Mail: JuntaranMail@gmail.com
    > Created Time: Tue 17 May 2016 20:20:12 PM CST
 ************************************************************************/

/*************************************************************************
    
    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.

 ************************************************************************/

#include <stdio.h>

int findDuplicate(int* nums, int numsSize)
{
    if(numsSize > 0)
    {
        int slow = nums[0];
        int fast = nums[nums[0]];
        
        while(slow != fast) //check the existence of the loop;
        {
            printf("%d %d
", slow, fast);
            slow = nums[slow];
            fast = nums[nums[fast]];
        }
        printf("%d %d
", slow, fast);
        
        fast = 0;
        while(slow != fast) //find the start of the loop which means at least two integer are the same value;
        {
            slow = nums[slow];
            fast = nums[fast];
        }
        return slow;
    }
    return -1;
}

int main()
{
    int nums[] = {1,3,4,2,2};
    int numsSize = 5;
    
    int ret = findDuplicate( nums, numsSize);
    printf("%d
", ret);
}