leetcode85 - Maximal Rectangle - hard

class Solution {
    public int maximalRectangle(char[][] matrix) {
        // invalid input.
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
            return 0;
        }
        
        int ans = 0;
        int[] heights = new int[matrix[0].length];
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix[0].length; j++) {
                if (matrix[i][j] == ‘0‘) {
                    heights[j] = 0;
                } else {
                    heights[j]++;
                }
            }
            ans = Math.max(ans, maxRecInHistogram(heights));
        }
        return ans;
    }
    
    private int maxRecInHistogram(int[] heights) {
        int ans = 0;
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i <= heights.length; i++) {
            while (!stack.isEmpty() && (i == heights.length || heights[i] < heights[stack.peek()])) {
                int height = heights[stack.pop()];
                int width = stack.isEmpty() ? i : i - stack.peek() - 1;
                ans = Math.max(ans, height * width);
            }
            stack.push(i);
        }
        return ans;
    }
}