LeetCode Maximal Rectangle

原题链接在这里：https://leetcode.com/problems/maximal-rectangle/

与Largest Rectangle in Histogram相似. 用一个数组dp存储一行中每一个点的最大高度，然后像Largest Rectangle in Histogram算这一行能产生的最大面积maxRec.

Time O(m*n), m是matrix的高，n是matrix的长。Space O(n).

AC Java:

public class Solution {
    public int maximalRectangle(char[][] matrix) {
        if(matrix == null || matrix.length == 0 || matrix[0].length == 0){
            return 0;
        }
        int h = matrix.length;
        int w = matrix[0].length;
        // length,length,length,length,length
        int [] dp = new int[w];
        int maxRec = 0;
        for(int i = 0; i<h; i++){
            for(int j=0; j<w; j++){
                dp[j] = matrix[i][j] == '0' ? 0 : dp[j]+1;
            }
            maxRec = Math.max(maxRec,helper(dp));
        }
        return maxRec;
    }
    private int helper(int [] nums){
        if(nums == null || nums.length == 0){
            return 0;
        }
        Stack<Integer> stk = new Stack<Integer>();
        int [] h = new int[nums.length + 1];
        h = Arrays.copyOf(nums, nums.length+1);
        int maxArea = 0;
        
        int i = 0;
        while(i<h.length){
            if(stk.isEmpty() || h[stk.peek()] <= h[i]){
                stk.push(i);
                i++;
            }else{
                int index = stk.pop();
                maxArea = Math.max(maxArea,h[index]*(stk.isEmpty() ? i : i-stk.peek()-1));
            }
        }
        return maxArea;
    }
}