Move Zeroes

LeetCode 283 | Difficulty: Easy

Easy

Problem Description

Given an integer array nums, move all 0's to the end of it while maintaining the relative order of the non-zero elements.

Note that you must do this in-place without making a copy of the array.

Example 1:

Input: nums = [0,1,0,3,12]
Output: [1,3,12,0,0]

Example 2:

Input: nums = [0]
Output: [0]

Constraints:

- `1 <= nums.length <= 10^4`

- `-2^31 <= nums[i] <= 2^31 - 1`

Follow up: Could you minimize the total number of operations done?

Topics: Array, Two Pointers

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Approach

Two Pointers

Use two pointers to traverse the array, reducing the search space at each step. This avoids the need for nested loops, bringing complexity from O(n²) to O(n) or O(n log n) if sorting is involved.

When to use

Array is sorted or can be sorted, and you need to find pairs/triplets that satisfy a condition.

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Solutions

Solution 1: C# (Best: 236 ms)

Metric	Value
Runtime	236 ms
Memory	31.8 MB
Date	2020-04-06

Solution

public class Solution {
    public void MoveZeroes(int[] nums) {
        int index = 0;
    for (int i = 0; i < nums.Length; i++)
    {
        if(nums[i]!=0)
        {
            if(index!=i){
            int  temp =  nums[index];
            nums[index] = nums[i];
            nums[i] = temp;
            }
            index++;
        }
        
    }
    }
}

📜 2 more C# submission(s)

Submission (2018-05-11) — 292 ms, N/A

public class Solution {
    public void MoveZeroes(int[] nums) {
        int index = 0;
    for (int i = 0; i < nums.Length; i++)
    {
        if(nums[i]!=0)
        {
            if(index!=i){
            int  temp =  nums[index];
            nums[index] = nums[i];
            nums[i] = temp;
            }
            index++;
        }
        
    }
    }
}

Submission (2018-05-11) — 292 ms, N/A

public class Solution {
    public void MoveZeroes(int[] nums) {
        int index = 0;
    for (int i = 0; i < nums.Length; i++)
    {
        if(nums[i]!=0)
        {
            int  temp =  nums[index];
            nums[index] = nums[i];
            nums[i] = temp;
            index++;
        }
        
    }
    }
}

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Complexity Analysis

Approach	Time	Space
Two Pointers	$O(n)$	$O(1)$

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Interview Tips

Key Points

• Start by clarifying edge cases: empty input, single element, all duplicates.
• Ask: "Can I sort the array?" — Sorting often enables two-pointer techniques.
• LeetCode provides 2 hint(s) for this problem — try solving without them first.

💡 Hints

Hint 1: In-place means we should not be allocating any space for extra array. But we are allowed to modify the existing array. However, as a first step, try coming up with a solution that makes use of additional space. For this problem as well, first apply the idea discussed using an additional array and the in-place solution will pop up eventually.

Hint 2: A two-pointer approach could be helpful here. The idea would be to have one pointer for iterating the array and another pointer that just works on the non-zero elements of the array.