Solutions for 2 questions on leetcode

The Questions are :
1st -> Largest Rectangle in Histogram
https://leetcode.com/problems/largest-rectangle-in-histogram/

2nd -> Valid Parenthesis
https://leetcode.com/problems/valid-parentheses/

LeetCode Solutions/LargestRectangleInHistogram.md

@@ -0,0 +1,62 @@
+class Solution {
+private:
+    vector<int> nextSmallerElement(vector<int> arr, int n) {
+        stack<int> s;
+        s.push(-1);
+        vector<int> ans(n);
+
+        for(int i=n-1; i>=0 ; i--) {
+            int curr = arr[i];
+            while(s.top() != -1 && arr[s.top()] >= curr)
+            {
+                s.pop();
+            }
+            //ans is stack ka top
+            ans[i] = s.top();
+            s.push(i);
+        }
+        return ans;
+    }
+
+    vector<int> prevSmallerElement(vector<int> arr, int n) {
+        stack<int> s;
+        s.push(-1);
+        vector<int> ans(n);
+
+        for(int i=0; i<n; i++) {
+            int curr = arr[i];
+            while(s.top() != -1 && arr[s.top()] >= curr)
+            {
+                s.pop();
+            }
+            //ans is stack ka top
+            ans[i] = s.top();
+            s.push(i);
+        }
+        return ans; 
+    }
+
+public:
+    int largestRectangleArea(vector<int>& heights) {
+        int n= heights.size();
+
+        vector<int> next(n);
+        next = nextSmallerElement(heights, n);
+
+        vector<int> prev(n);
+        prev = prevSmallerElement(heights, n);
+
+        int area = INT_MIN;
+        for(int i=0; i<n; i++) {
+            int l = heights[i];
+
+            if(next[i] == -1) {
+                next[i] = n;
+            }
+             int b = next[i] - prev[i] - 1;
+            int newArea = l*b;
+            area = max(area, newArea);
+        }
+        return area;
+    }
+};

LeetCode Solutions/ValidParenthesis.md

@@ -0,0 +1,42 @@
+class Solution {
+public:
+    bool isValid(string expression) {
+  	 stack<char> s;
+     for(int i=0; i<expression.length(); i++) {
+
+         char ch = expression[i];
+
+         //if opening bracket, stack push
+         //if close bracket, stacktop check and pop
+
+         if(ch == '(' || ch == '{' || ch == '['){
+             s.push(ch);
+         }
+         else
+         {
+             //for closing bracket
+             if(!s.empty()) {
+                  char top = s.top();
+                  if( (ch == ')' && top == '(') || 
+                     ( ch == '}' && top == '{') || 
+                     (ch == ']' && top == '[') ) {
+                      s.pop();
+                  }
+                 else
+                 {
+                     return false;
+                 }
+             }
+             else
+             {
+                 return false;
+             } 
+         }  
+     }
+
+    if(s.empty())
+        return true;
+    else
+        return false;
+}
+};