LeetCode 85. Maximal Rectangle --python,java解法

LeetCode 85. Maximal Rectangle

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LeetCode题解专栏：LeetCode题解
我做的所有的LeetCode的题目都放在这个专栏里，大部分题目Java和Python的解法都有。

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

Example:

Input:
[
  ["1","0","1","0","0"],
  ["1","0","1","1","1"],
  ["1","1","1","1","1"],
  ["1","0","0","1","0"]
]
Output: 6

这道题的一个巧妙的思路是基于这道题的：LeetCode 84. Largest Rectangle in Histogram
Python解法如下：

class Solution:

    # Get the maximum area in a histogram given its heights
    def leetcode84(self, heights):
        stack = [-1]

        maxarea = 0
        for i in range(len(heights)):

            while stack[-1] != -1 and heights[stack[-1]] >= heights[i]:
                maxarea = max(maxarea, heights[stack.pop()] * (i - stack[-1] - 1))
            stack.append(i)

        while stack[-1] != -1:
            maxarea = max(maxarea, heights[stack.pop()] * (len(heights) - stack[-1] - 1))
        return maxarea


    def maximalRectangle(self, matrix: List[List[str]]) -> int:

        if not matrix: return 0

        maxarea = 0
        dp = [0] * len(matrix[0])
        for i in range(len(matrix)):
            for j in range(len(matrix[0])):

                # update the state of this row's histogram using the last row's histogram
                # by keeping track of the number of consecutive ones

                dp[j] = dp[j] + 1 if matrix[i][j] == '1' else 0

            # update maxarea with the maximum area from this row's histogram
            maxarea = max(maxarea, self.leetcode84(dp))
        return maxarea

java解法如下：

class Solution {

    // Get the maximum area in a histogram given its heights
    public int leetcode84(int[] heights) {
        Stack < Integer > stack = new Stack < > ();
        stack.push(-1);
        int maxarea = 0;
        for (int i = 0; i < heights.length; ++i) {
            while (stack.peek() != -1 && heights[stack.peek()] >= heights[i])
                maxarea = Math.max(maxarea, heights[stack.pop()] * (i - stack.peek() - 1));
            stack.push(i);
        }
        while (stack.peek() != -1)
            maxarea = Math.max(maxarea, heights[stack.pop()] * (heights.length - stack.peek() -1));
        return maxarea;
    }

    public int maximalRectangle(char[][] matrix) {

        if (matrix.length == 0) return 0;
        int maxarea = 0;
        int[] dp = new int[matrix[0].length];

        for(int i = 0; i < matrix.length; i++) {
            for(int j = 0; j < matrix[0].length; j++) {

                // update the state of this row's histogram using the last row's histogram
                // by keeping track of the number of consecutive ones

                dp[j] = matrix[i][j] == '1' ? dp[j] + 1 : 0;
            }
            // update maxarea with the maximum area from this row's histogram
            maxarea = Math.max(maxarea, leetcode84(dp));
        } return maxarea;
    }
}