84. Largest Rectangle in Histogram

    /*
     * 84. Largest Rectangle in Histogram 
     * 2016-5-13 by Mingyang
     * 这里并不需要两个stack，只需要一个stack，装的是递增序列的index
     * 直到遇到一个递减的时候，弹出来，求一个一个的面积大小
     * 不过注意的是最后如果以递增的序列结尾的话，还需要一个一个的求完
     */
    public int largestRectangleArea(int[] height) {
        if (height == null || height.length == 0) {
            return 0;
        } 
        Stack<Integer> stack = new Stack<Integer>();     
        int max = 0;
        int i = 0; 
        while (i < height.length) {
            //push index to stack when the current height is larger than the previous one
            if (stack.isEmpty() || height[i] >= height[stack.peek()]) {
                stack.push(i);
                i++;
            } else {
            //calculate max value when the current height is less than the previous one
                int p = stack.pop();
                int h = height[p];
                int w = stack.isEmpty() ? i : i - stack.peek() - 1;
                max = Math.max(h * w, max);
            }     
        } 
        while (!stack.isEmpty()) {
            int p = stack.pop();
            int h = height[p];
            int w = stack.isEmpty() ? i : i - stack.peek() - 1;
            max = Math.max(h * w, max);
        } 
        return max;
    }