Create largestRectangleInHistogram.cpp

Solved Leetcode Hard Problem 84. Largest Rectangle in Histogram

largestRectangleInHistogram.cpp

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+class Solution {
+private:
+    vector<int> nextSmallerElements(vector<int> &heights,int n){
+        stack<int> st;
+        st.push(-1);
+        vector<int> ans(n);
+
+        for(int i=n-1; i>=0; i--){
+            while(st.top()!= -1 && heights[st.top()]>=heights[i]){
+                st.pop();
+            }
+            ans[i] = st.top();
+            st.push(i);
+        }
+
+        return ans;
+    }
+
+    vector<int> prevSmallerElements(vector<int> &heights,int n){
+        stack<int> st;
+        st.push(-1);
+        vector<int> ans(n);
+
+        for(int i=0; i<n; i++){
+            while(st.top()!= -1 && heights[st.top()]>=heights[i]){
+                st.pop();
+            }
+            ans[i] = st.top();
+            st.push(i);
+        }
+
+        return ans;
+    }
+public:
+    int largestRectangleArea(vector<int>& heights) {
+        int n = heights.size();
+
+        vector<int> next(n);
+        next = nextSmallerElements(heights, n);//indices
+
+        vector<int> prev(n);
+        prev = prevSmallerElements(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;
+            area = max(area, l*b);
+        }
+
+        return area;
+    }
+};