84.js

/**
 * @param {number[]} heights
 * @return {number}
 */
function largestRectangleArea(heights) {
    heights.push(-1);
    var stack = [];
    var max = 0;
    
    heights.forEach((h, i) => {
        if (!stack.length || heights[stack.slice(-1)[0]] < h) {
            return stack.push(i);
        }
        
        while (stack.length && heights[stack.slice(-1)[0]] >= h) {
            var height = heights[stack.pop()];
            var left = stack.length ? stack.slice(-1)[0] + 1 : 0;
            
            max = Math.max(max, height * (i - 1 - left + 1));
        }
        stack.push(i);
    });
    
    return max;
}