This is the extension of Largest Rectangle in Histogram. We can just project 2D matrix to 1D array and compute it line by line.

1 class Solution { 2 public: 3     int maximalRectangle(vector
     
      
        > &matrix) { 4         if (matrix.size() == 0) return 0; 5         int n = matrix.size(), m = matrix[0].size(), result = 0; 6         vector
       
         height(m, 0), left(m), right(m); 7         for (int i = 0; i < n; i++) { 8             for (int j = 0; j < m; j++) { 9                 if (matrix[i][j] == '1') height[j]++;10                 else height[j] = 0;11             }12             for (int j = 0; j < m; j++) {13                 left[j] = j;14                 while (left[j] > 0 && height[j] <= height[left[j]-1]) left[j] = left[left[j]-1];15             }16             for (int j = m-1; j >= 0; j--) {17                 right[j] = j;18                 while (right[j] < m-1 && height[j] <= height[right[j]+1]) right[j] = right[right[j]+1];19             }20             for (int j = 0; j < m; j++) {21                 result = max(result, height[j]*(right[j] - left[j] + 1));22             }23         }24         return result;25     }26 };