Added the largest rectangle in histogram problem

Uncategorized/Largest Rectangle In Histogram/README.md

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+# Largest Rectangle In Histogram
+
+Given an array of integers heights representing the histogram's bar height where the width of each bar is 1, return the area of the largest rectangle in the histogram.
+
+## Example:
+![plot](./histogram.jpg?raw=true)
+
+Input: heights = [2,1,5,6,2,3]
+Output: 10
+Explanation: The above is a histogram where width of each bar is 1.
+The largest rectangle is shown in the red area, which has an area = 10 units.

Uncategorized/Largest Rectangle In Histogram/largest.cpp

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+#include <iostream>
+#include <vector>
+#include <stack>
+using namespace std;
+
+vector<int> nearestSmallerOnRight(vector<int>& heights) {
+    stack<int>st;
+    vector<int>ans(heights.size(),heights.size());
+    for(int i=0;i<heights.size();i++)
+    {
+        while(!st.empty() && heights[i]<heights[st.top()])
+        {
+            ans[st.top()]=i;
+            st.pop();
+        }
+        st.push(i);
+    }  
+    return ans;
+}
+
+vector<int> nearestSmallerOnLeft(vector<int>& heights)
+{
+    stack<int>st;
+    vector<int>ans(heights.size(),-1);
+    for(int i=heights.size()-1;i>=0;i--)
+    {
+        while(!st.empty() && heights[i]<heights[st.top()])
+        {
+            ans[st.top()]=i;
+            st.pop();
+        }
+        st.push(i);
+    }
+    return ans;
+}
+
+int largestRectangleArea(vector<int>& heights)
+{
+    vector<int>NSR=nearestSmallerOnRight(heights);
+    vector<int>NSL=nearestSmallerOnLeft(heights);
+    int ans=0;
+    for(int i=0;i<heights.size();i++)
+        ans=max(ans,(NSR[i]-NSL[i]-1)*heights[i]);
+    return ans;
+}
+
+int main() {
+    vector<int> vec = {2, 1, 5, 6, 2, 3};
+    cout << largestRectangleArea(vec);
+}