added leetcode maximal rectangle problem

C++/Maximal_rectangle.cpp

@@ -0,0 +1,84 @@
+//Problem: https://leetcode.com/problems/maximal-rectangle/
+
+//Solution
+#include<iostream>
+#include<vector>
+#include<stack>
+using namespace std;
+
+class Solution {
+public:
+    vector<int> nxtSmaller(vector<int> n){
+        int sz = n.size();
+        vector<int> res(sz,-1);
+        stack<int> s;
+        for(int i=sz-1;i>=0;i--){
+            if(s.empty())
+                s.push(i);
+            else{
+                while(!s.empty() && n[s.top()]>=n[i])
+                    s.pop();
+                res[i] = !s.empty() ? s.top() : -1;
+                s.push(i);
+            }
+        }
+        return res;
+    }
+    vector<int> prevSmaller(vector<int> n){
+        int sz = n.size();
+        vector<int> res(sz,-1);
+        stack<int> s;
+        for(int i=0;i<sz;i++){
+            if(s.empty())
+                s.push(i);
+            else{
+                while(!s.empty() && n[s.top()]>=n[i])
+                    s.pop();
+                res[i] = !s.empty() ? s.top() : -1;
+                s.push(i);
+            }
+        }
+        return res;
+    }
+    int largestArea(vector<int> num){
+        int area,maxArea = INT_MIN;
+        int n = num.size();
+        vector<int> nxt = nxtSmaller(num);
+        vector<int> prev = prevSmaller(num);
+        for(int i=0;i<n;i++){
+            int h = num[i];
+            if(nxt[i]==-1){
+                nxt[i] = n;
+            }
+            int b = nxt[i]-prev[i]-1;
+            area = h * b;
+            maxArea = max(maxArea,area);
+        }
+        return maxArea;
+    }
+    int maximalRectangle(vector<vector<char>>& m) {
+        vector<vector<int>> matrix(m.size(),vector<int>(m[0].size(),0));
+        for(int i=0;i<m.size();i++){
+            for(int j=0;j<m[0].size();j++){
+                if(m[i][j]=='1'){
+                    matrix[i][j] = 1;
+                }
+            }
+        }
+        int area;
+        int maxArea = largestArea(matrix[0]);
+        for(int i=1;i<matrix.size();i++){
+            for(int j=0;j<matrix[0].size();j++){
+                if(matrix[i][j]!= 0){
+                    matrix[i][j] += matrix[i-1][j];
+                }
+                else{
+                    matrix[i][j] = 0;
+                }
+            }
+            area = largestArea(matrix[i]);
+            maxArea = max(area,maxArea);
+        }
+        return maxArea;
+    }
+};