Chapters
Top 5 Array Interview Problems in Java (With Real-World Solutions)
Problem #2: Move All Zeroes to the End of the Array (In-Place)
🧠 Problem Statement
Given:
An array of integers.
Task:
Move all the 0s to the end of the array while maintaining the relative order
of non-zero elements.
Do this in-place without making a copy of the array.
🧾 Sample Input/Output
Input:
int[] arr = {0, 1, 0, 3, 12};
Output:
[1, 3, 12, 0, 0]
Input:
int[] arr = {1, 2, 0, 0, 4, 5};
Output:
[1, 2, 4, 5, 0, 0]
🔍 Naive Approach: Using
Extra Array
This violates the “in-place” constraint — but it’s easy to
start with.
public
static int[] moveZeroesCopy(int[] arr) {
int[] result = new int[arr.length];
int index = 0;
for (int num : arr) {
if (num != 0) {
result[index++] = num;
}
}
// zeros are already 0 by default in new
array
return result;
}
⛔ Space Complexity: O(n)
✅ Optimal In-Place Solution (Two
Pointers)
Use one pointer to track the current position to place
non-zero values.
public
static void moveZeroes(int[] arr) {
int index = 0;
// Step 1: Move all non-zero values forward
for (int i = 0; i < arr.length; i++) {
if (arr[i] != 0) {
arr[index++] = arr[i];
}
}
// Step 2: Fill the rest with zeroes
while (index < arr.length) {
arr[index++] = 0;
}
}
🧠 Explanation
- Iterate
and copy non-zero values to the front (index keeps track).
- After
the loop, fill the rest of the array with zeroes starting from index.
🔁 Real Interview
Follow-Up: One-Pass Swap
public
static void moveZeroesSwap(int[] arr) {
int lastNonZeroFoundAt = 0;
for (int i = 0; i < arr.length; i++) {
if (arr[i] != 0) {
int temp = arr[lastNonZeroFoundAt];
arr[lastNonZeroFoundAt] = arr[i];
arr[i] = temp;
lastNonZeroFoundAt++;
}
}
}
✅ Preserves relative order
✅
Swaps in-place
✅
One-pass
⚠️ Edge Cases to Consider
|
Input |
Output |
Notes |
|
[0, 0, 0] |
[0, 0, 0] |
All zeroes |
|
[1, 2, 3] |
[1, 2, 3] |
No change needed |
|
[] |
[] |
Empty array |
|
[0, 1] |
[1, 0] |
Smallest non-trivial case |
📈 Time & Space
Complexity
|
Operation |
Value |
|
Time |
O(n) |
|
Space |
O(1) — In-place |
🧨 Interview Variants
- "Move
all zeroes to the front instead"
Reverse logic — fill from back to front - "Do
it in a single pass without using extra variables"
Use XOR swap trick (not preferred for clarity) - "Count
minimum number of swaps to move zeroes to end"
Count how many times zero and non-zero indices swap
✅ Summary
|
Step |
Description |
|
Two-pointer strategy |
Place non-zero at next available slot |
|
In-place operation |
No extra array required |
|
One-pass swap optimization |
Faster for large arrays |
|
Preserves order |
Interviewer will often test for this too |
FAQs
1. Why are array problems so common in Java interviews?
Because
they test core programming logic, data handling, loops, and algorithm
efficiency.
2. What’s the difference between int[] and Integer[] in Java?
int[] is a primitive array; Integer[] is an array of objects (wrappers). The latter allows null values and works with collections.
3. Is it better to use Arrays or ArrayLists in interviews?
For fixed-size problems, use arrays. For dynamic data, ArrayList is better — but stick to arrays unless otherwise asked
4. What are common pitfalls in Java array questions?
Index out of bounds, mutating arrays while iterating, and forgetting that Java arrays have fixed size.
5. How do I remove duplicates from a Java array?
Use a Set for uniqueness or sort the array first and remove duplicates in-place using two pointers.
6. When should I use the two-pointer technique?
It’s great for sorted arrays, especially for problems involving pair sums, removing duplicates, and reversing data in-place.
7. How can I rotate an array in Java?
Use reversal techniques or cyclic replacements to do it in O(1) space.
8. What’s the time complexity of array search vs binary search?
Linear search = O(n); binary search = O(log n), but only on sorted arrays.
9. How do I handle negative numbers or large input arrays?
Always consider edge cases: empty arrays, one element, all duplicates, etc. Optimize with hashmaps or prefix sums where needed.
10. What libraries or classes help with arrays in Java?
Use Arrays.sort(), System.arraycopy(), Arrays.toString(), and Collections when applicable — but show the manual solution first in interviews.
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