Stack • Monotonic Stack

Previous Greater Element (PGE)

Given an array, find the first strictly greater element to the left of each element. This relies on a Monotonic Decreasing Stack but traversed Left-to-Right.

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The Direction Change

For NEXT Greater Element, we usually iterate right-to-left. For PREVIOUS Greater Element, we naturally iterate left-to-right, because the history we care about is on the left.

Decreasing Stack

By popping any element that is `<=` the current element, we ensure the stack strictly decreases from bottom to top. The top of the stack is always the closest greater element.

Core Concept

If an element `A` is smaller than the current element `B`, `A` can NEVER be the "Previous Greater" for any future elements, because `B` blocks it. Thus, it's safe to pop `A` forever.

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Top row: Array | Bottom row: PGE Result

Monotonic Decreasing Stack

Empty

Code Tracker

:::

1let stack = []; let result = new Array(n).fill(-1);

2for (let i = 0; i < n; i++) {

3 while (stack.length > 0 && stack[stack.length - 1] <= nums[i]) {

4 stack.pop();

5 }

6 if (stack.length > 0) {

7 result[i] = stack[stack.length - 1];

8 }

9 stack.push(nums[i]);

10}

11return result;