LeetCode - Maximal Rectangle

題目：

Given a 2D binary matrix filled with 0's and 1's, find the largest rectangle containing all ones and return its area.

思路：

類似於上一篇文章中的方法一，對每列的左右拓展極限進行記錄，同時保存當前列的高度。

package area;

public class MaximalRectangle {

    public int maximalRectangle(char[][] matrix) {
        int m;
        int n;
        if (matrix == null || (m = matrix.length) == 0 || (n = matrix[0].length) == 0) return 0;
        // Height of this column
        int[] H = new int[n];
        int[] L = new int[n];
        int[] R = new int[n];
        for (int i = 0; i < n; ++i) {
            H[i] = 0;
            L[i] = 0;
            R[i] = n;
        }
        int max = 0;
        for (int i = 0; i < m; ++i) {
            int left = 0;
            int right = n;
            for (int j = 0; j < n; ++j) {
                if (matrix[i][j] == '1') {
                    ++H[j];
                    L[j] = Math.max(L[j], left);
                } else {
                    left = j + 1;
                    H[j] = 0;
                    L[j] = 0;
                    R[j] = n;
                }
            }
            
            for (int j = n - 1; j >= 0; --j) {
                if (matrix[i][j] == '1') {
                    R[j] = Math.min(R[j], right);
                    max = Math.max((R[j] - L[j]) * H[j], max);
                } else {
                    right = j;
                }
            }
        }
        return max;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        char[][] matrix = { { '0', '1', '1', '0', '1' },
                { '1', '1', '0', '1', '0' }, { '0', '1', '1', '1', '0' },
                { '1', '1', '1', '1', '0' }, { '1', '1', '1', '1', '1' },
                { '0', '0', '0', '0', '0' } };
        MaximalRectangle m = new MaximalRectangle();
        System.out.println(m.maximalRectangle(matrix));
    }

}