[LeetCode 84] Largest Rectangle in Histogram
Given n non-negative integers representing the histogram's bar height where the width of each bar is 1, find the area of largest rectangle in the histogram.
Above is a histogram where width of each bar is 1, given height = [2,1,5,6,2,3].
The largest rectangle is shown in the shaded area, which has area = 10 unit.
Example:
Input: [2,1,5,6,2,3]
Output: 10
Analysis:
Q: Think about how you would solve this problem by hand?
A: Given the rectangle requirement, for two consecutive heights height[i] and height[i + 1], if height[i + 1] < height[i], we know that the rectangle whose heights are height[i] has just reached their right ends as they can not use height[i + 1] anymore. So we calculate the area of all rectangles whose right ends are index i and heights are height[i] . If height[i + 1] >= height[i], then we do not do any calculations becase we are not sure about the right bound of any rectangles.
This implies maintaining a non-decreasing stack of heights. We keep pushing bigger or the the same heights into this stack until we hit a smaller height than previous. Then we keep poping heights that are strictly bigger than the current height and calculate rectangle areas until we hit the left boundary(top of the stack is <= the current height).
Repeat the above process until we've iterated all heights. Any heights left in the stack are non-decreasing, pop these indices out and calcuate their corresponding rectangle areas. To make the calculation easier, instead of storing the actual height in the stack, their indices in the array are stored. This way it is easier to calculate the length of each rectangle by probing the top of the stack.
O(n) runtime, O(n) space
class Solution {
public int largestRectangleArea(int[] heights) {
int ans = 0, i = 0;
Stack<Integer> stack = new Stack<>();
stack.push(-1);
while(i < heights.length) {
while(stack.peek() >= 0 && heights[stack.peek()] > heights[i]) {
int j = stack.pop();
ans = Math.max(ans, heights[j] * (i - 1 - stack.peek()));
}
stack.push(i);
i++;
}
while(stack.peek() >= 0) {
int j = stack.pop();
ans = Math.max(ans, heights[j] * (i - 1 - stack.peek()));
}
return ans;
}
}
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