0085._Maximal_Rectangle.md

85. Maximal Rectangle

难度:Hard

刷题内容

原题连接

• https://leetcode.com/problems/maximal-rectangle/

内容描述

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

思路1 - 时间复杂度: O(n^3)- 空间复杂度: O(1)******

第一种暴力的方法去解，先求出每一个数向右有多少个“1”，记录下长度count1，在这个数的位置向下寻找，若下面的长度小于count1，则count1取较小的值。若为0就计算矩阵的大小，直到遍历所有的数。

class Solution {
public:
    int maximalRectangle(vector<vector<char>>& matrix) {
        int len1 = matrix.size();
        if(!len1)
            return 0;
        int len2 = matrix[0].size();
        int ans = 0;
        for(int t = 0;t < len1;++t)
        for(int i = 0;i < len2;++i)
        {
            int count1 = 0;
            while(i + count1 < len2 && matrix[t][i + count1] - '0')
                ++count1;
            if(count1)
            {
                int row = 1;
                ans = max(ans,row * count1);
                for(int j = t + 1;j < len1;++j)
                {
                    int count2 = 0;
                    while(count2  <= count1 && matrix[j][i + count2] - '0')
                        ++count2;
                    if(!count2)
                        break;
                    if(count1 > count2)
                        count1 = count2;
                    ++row;
                    ans = max(ans,row * count1);
                }
            }
        }
    return ans;
    }
};

思路2 - 时间复杂度: O(n^2)- 空间复杂度: O(n)******

对上述算法进行优化，计算每个数的高度和这个高度向左的位置和向右的位置，这样只要在O(n^2)的时间复杂内就能完成

class Solution {
public:
    int maximalRectangle(vector<vector<char>>& matrix) {
              int len1 = matrix.size();
        if(!len1)
            return 0;
        int len2 = matrix[0].size();
        int height[len2],l[len2],r[len2];
        memset(height,0,sizeof(height));
        memset(l,0,sizeof(l));
        for(int i = 0;i < len2;++i)
            r[i]  = len2;
        int ans = 0;
        for(int i = 0;i < len1;++i)
        {
            int t_l = 0,t_r = len2;
            for(int j = 0;j < len2;++j)
                if(matrix[i][j]-'0')
                    height[j] = height[j] + 1;
                else
                    height[j] = 0;
            for(int j = 0;j < len2;++j)
                if(matrix[i][j]-'0')
                    l[j] = max(l[j],t_l);
                else
                {
                    t_l = j + 1;
                    l[j] = 0;
                }
            for(int j = len2 - 1;j >= 0;--j)
                if(matrix[i][j]-'0')
                    r[j] = min(r[j],t_r);
                else
                {
                    t_r = j;
                    r[j] = len2;
                }
            for(int j = 0;j < len2;++j)
                ans = max(ans,(r[j] - l[j]) * height[j]);
        }
        return ans;
    }
};