解题思路：

单调栈

对于一个高度height[ i ]，找左右两边均严格小于它的值。

class Solution {public int largestRectangleArea(int[] heights) {int n = heights.length;int[] left = new int[n];int[] right = new int[n];Deque<Integer> mono_stack = new ArrayDeque<>();for (int i = 0; i < n; i++) {while (!mono_stack.isEmpty() && heights[mono_stack.peek()] >= heights[i]) {mono_stack.pop();}left[i] = (mono_stack.isEmpty() ? -1 : mono_stack.peek());mono_stack.push(i);}mono_stack.clear();for (int i = n - 1; i >= 0; i--) {while (!mono_stack.isEmpty() && heights[mono_stack.peek()] >= heights[i]) {mono_stack.pop();}right[i] = (mono_stack.isEmpty() ? n : mono_stack.peek());mono_stack.push(i);}int ans = 0;for (int i = 0; i < n; i++) {ans = Math.max(ans, (right[i] - left[i] - 1) * heights[i]);}return ans;}
}