added largest-rectangle-in-histogram.java code

Author: pranav-3005

Coding/Java/largest-rectangle-in-histogram.java

@@ -0,0 +1,65 @@
+//Question : https://leetcode.com/problems/largest-rectangle-in-histogram/description/
+
+class Solution {
+    public int largestRectangleArea(int[] h) {
+
+        //monotonic stack
+        
+        int n=h.length;
+        int left[]=new int[n];  //left small elem's ind'
+        int ryt[]=new int[n];  //ryt smaller elem's ind'
+        
+        Stack<Integer> lin=new Stack<>(); //storing index in stacks
+        Stack<Integer> rin=new Stack<>();
+        
+        int ans[]=new int[n]; //ans array
+
+        //1.store each elem's ryt side next small elem index in ryt[]
+        for(int i=0;i<n;i++)
+        {
+            //before add cur elem index to stack, 
+            //we check if its the ryt next small of elems in stack
+            while( rin.size()>0 && h[i]< h[rin.peek()] )
+            {
+                ryt[rin.peek()]=i;
+                rin.pop();
+            }
+            rin.push(i);
+        }   
+        //if no small elem present in ryt, assign n.
+        while(rin.size()>0) 
+        {
+            ryt[rin.pop()]=n;
+        }
+
+
+        //2.store each elem's left side next small elem index in left[]
+        for(int i=n-1;i>=0;i--)
+        {
+            //before add cur elem index to stack, 
+            //we check if its the left next small of elems in stack
+            while( lin.size()>0 && h[i]<h[lin.peek()] )
+            {
+                left[lin.pop()]=i;
+            }
+            lin.push(i);
+        }
+        //if no small elem present in left, assign '-1'.
+        while(lin.size()>0)
+        {
+            left[lin.pop()]=-1;
+        }
+
+
+        //3. calculate area of rectangle(i.e area=width*height)
+        //store ans arr in left[]  //height=value at an ind
+        for(int i=0;i<n;i++)
+        {
+            int w=ryt[i]-left[i] - 1; //width
+            ans[i]= w * h[i] ; //area
+        }
+
+        //4. return max of ans arr(i.e...left[])
+        return Arrays.stream(ans).max().getAsInt();
+    }
+}