84. Largest Rectangle in Histogram

Author: SjxSubham

84. Largest Rectangle in Histogram

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+//here we have to learn two more quentions first ...
+//Which are most important in Stack
+// 1.next Smaller Element
+//2. prev Smaller Element
+
+//then we will be able to solve it
+//Formula----
+// area = length * width;
+
+
+class Solution {
+public:
+    vector<int>nextSmallerElement(vector<int>& input){
+        stack<int>s;
+        s.push(-1);
+        vector<int>ans(input.size());
+        for(int i=input.size()-1;i>=0;i--){
+            int curr = input[i];
+            while(s.top()!= -1 && input[s.top()] >= curr){
+                s.pop();
+            }
+            //chota mil chuka hai store ans
+            ans[i] = s.top();
+            //push kardo element ko
+            s.push(i);
+        }
+        return ans;
+    }
+    vector<int>prevSmallerElement(vector<int>& input){
+        stack<int>s;
+        s.push(-1);
+        vector<int>ans(input.size());
+        //left to right
+        for(int i=0;i<input.size();i++){
+            int curr = input[i];
+        
+        while(s.top()!= -1 && input[s.top()] >= curr){
+            s.pop();
+        }
+        //chota element mil chuka hai
+        ans[i] = s.top();
+        //push kardo elment ko
+        s.push(i);
+        }
+        return ans;
+    }
+    int largestRectangleArea(vector<int>& heights) {
+        //prev smaller call
+       vector<int>prev =  prevSmallerElement(heights);
+       //next smaller call
+       vector<int>next =  nextSmallerElement(heights);
+        int MaxArea = INT_MIN;
+        int size = heights.size();
+
+        for(int i=0;i<heights.size();i++){
+            int length = heights[i];
+
+            if(next[i] == -1){
+                next[i] = size;
+            }
+            int width = next[i] - prev[i] -1;
+            int area = length * width;
+            MaxArea = max(MaxArea,area);
+        }
+        return MaxArea;
+    }
+};