Challenge - 5 Problems
Next Greater Element Master
Get all challenges correct to earn this badge!
Test your skills under time pressure!
❓ Predict Output
intermediate2:00remaining
Output of Next Greater Element for a simple array
What is the output of the following JavaScript code that finds the next greater element for each item in the array?
DSA Javascript
function nextGreaterElement(arr) {
const stack = [];
const result = new Array(arr.length).fill(-1);
for (let i = 0; i < arr.length; i++) {
while (stack.length && arr[stack[stack.length - 1]] < arr[i]) {
const idx = stack.pop();
result[idx] = arr[i];
}
stack.push(i);
}
return result;
}
console.log(nextGreaterElement([2, 1, 3]));Attempts:
2 left
💡 Hint
Think about how the stack helps track indices waiting for a bigger number.
✗ Incorrect
The code uses a stack to keep indices of elements for which the next greater element is not found yet. When a bigger element is found, it updates the result for all smaller elements on the stack.
❓ Predict Output
intermediate2:00remaining
Next Greater Element output for descending array
What will be the output of the next greater element function when the input array is strictly descending?
DSA Javascript
function nextGreaterElement(arr) {
const stack = [];
const result = new Array(arr.length).fill(-1);
for (let i = 0; i < arr.length; i++) {
while (stack.length && arr[stack[stack.length - 1]] < arr[i]) {
const idx = stack.pop();
result[idx] = arr[i];
}
stack.push(i);
}
return result;
}
console.log(nextGreaterElement([5, 4, 3, 2, 1]));Attempts:
2 left
💡 Hint
In a descending array, no element has a greater element to its right.
✗ Incorrect
Since the array is strictly descending, no element to the right is greater than the current element, so all results remain -1.
🔧 Debug
advanced2:00remaining
Identify the error in Next Greater Element implementation
What error will the following code produce when run, and why?
DSA Javascript
function nextGreaterElement(arr) {
const stack = [];
const result = new Array(arr.length).fill(-1);
for (let i = 0; i <= arr.length; i++) {
while (stack.length && arr[stack[stack.length - 1]] < arr[i]) {
const idx = stack.pop();
result[idx] = arr[i];
}
stack.push(i);
}
return result;
}
console.log(nextGreaterElement([1, 2, 3]));Attempts:
2 left
💡 Hint
Check the loop boundary and array access inside the loop.
✗ Incorrect
The loop runs one step too far (i <= arr.length), causing arr[i] to be undefined on the last iteration, leading to a TypeError when comparing.
🧠 Conceptual
advanced2:00remaining
Stack usage in Next Greater Element algorithm
Why does the Next Greater Element algorithm use a stack to store indices instead of values directly?
Attempts:
2 left
💡 Hint
Think about how the result array is updated.
✗ Incorrect
Storing indices allows the algorithm to update the result array at the exact position corresponding to the element whose next greater element is found.
❓ Predict Output
expert3:00remaining
Next Greater Element output for circular array
What is the output of the following code that finds the next greater element in a circular array?
DSA Javascript
function nextGreaterElementCircular(arr) {
const n = arr.length;
const result = new Array(n).fill(-1);
const stack = [];
for (let i = 0; i < 2 * n; i++) {
const current = arr[i % n];
while (stack.length && arr[stack[stack.length - 1]] < current) {
const idx = stack.pop();
result[idx] = current;
}
if (i < n) stack.push(i);
}
return result;
}
console.log(nextGreaterElementCircular([1, 2, 1]));Attempts:
2 left
💡 Hint
Remember the array is circular, so elements after the end wrap around to the start.
✗ Incorrect
The circular approach allows elements at the end to find next greater elements at the start. For example, the last 1 finds 2 as next greater element from the start.