#283 Move Zeroes

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the problem

In plain words

Move all 0s to the end of the array while keeping the relative order of the non-zero elements. Do it in place.

in nums = [0,1,0,3,12]

out [1,3,12,0,0]

in nums = [0]

out [0]

1 ≤ n ≤ 10⁴

your turn

Try it before you peek

Read the examples, say the brute force out loud, then write code: here or on LeetCode ↗. Stuck for 20 minutes? Open one hint at a time.

Solve it in the browser

LeetCode-style: complete the class. Compiled by the real javac (Java 17) and judged right here — nothing is sent anywhere.

Solutions are hidden so you can try first. Even 20 minutes of real effort makes the answer stick far better than reading it.

Marking the problem solved, or opening every hint, also shows it.

the aha

Key insight

patternTwo pointers (read / write)

Pack non-zeros to the front with a write pointer; swapping (instead of copying) makes the zeros end up at the back automatically.

solutions

From brute force to optimal

Copy non-zeros into a fresh array (rest stays 0), then copy back.

copy = new int[n].
Append non-zeros.
Copy back.

timeO(n)

Two passes.

spaceO(n)

The extra array — violates "in place".

java

1class Solution {2    public void moveZeroes(int[] nums) {3        int[] copy = new int[nums.length];          // zeros by default4        int k = 0;5        for (int x : nums) if (x != 0) copy[k++] = x;6        System.arraycopy(copy, 0, nums, 0, nums.length);7    }8}

When nums[read] ≠ 0, swap it with nums[write] and advance write.

write = 0.
For each read: if non-zero, swap(read, write), write++.

timeO(n)

One pass.

spaceO(1)

In place.

java

1class Solution {2    public void moveZeroes(int[] nums) {3        int write = 0;4        for (int read = 0; read < nums.length; read++) {5            if (nums[read] != 0) {6                int tmp = nums[write];               // swap keeps zeros flowing right7                nums[write] = nums[read];8                nums[read] = tmp;9                write++;10            }11        }12    }13}

sim

Dry run

loading simulation…

java

1class Solution {2    public void moveZeroes(int[] nums) {3        int write = 0;4        for (int read = 0; read < nums.length; read++) {5            if (nums[read] != 0) {6                int tmp = nums[write];               // swap keeps zeros flowing right7                nums[write] = nums[read];8                nums[read] = tmp;9                write++;10            }11        }12    }13}

in the room

How to say it in the interview

talk track

Confirm: in place, and the non-zero values must keep their order. Negatives count as non-zero.
Brute: copy the non-zeros into a new array and copy back — O(n) time, but O(n) extra space.
Optimal: a write pointer for the next non-zero slot; swap each non-zero into it, and the zeros drift to the back.
State it: O(n) time, O(1) space. Test [0], an array with no zeros, and [0, 0, 1].

traps

Sorting the array — that breaks the original order of the non-zeros.
Removing zeros with ArrayList.remove and adding them at the end: O(n²) and not in place.
Copying non-zeros forward but forgetting to fill the tail with zeros afterwards.

what they ask next

Follow-ups

Can you reduce the number of writes?

Copy non-zeros forward (nums[write++] = nums[read]), then fill the rest with 0. Or keep the swap but skip it when read == write. Both stay O(n) time, O(1) space.

Move all copies of some value val instead of 0?

Same loop: swap forward every element that isn't val. It's LC 27 Remove Element, but keeping order.

before you submit

Edge cases to test

Only zeros, or no zeros at all: the array must stay as it is.
A single element [0] → [0].
Negative numbers are non-zero too: [0, -2] → [-2, 0].
Order matters: [4, 0, -2, 0, 7] → [4, -2, 7, 0, 0], not sorted.

practise next