Create 84. Largest Rectangle in Histogram

Author: khushi3004

84. Largest Rectangle in Histogram

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+class Solution {
+    public int largestRectangleArea(int[] h) {
+        //Check null condition
+        if(h == null || h.length == 0) return 0;
+        
+        //Initialize variables left and right
+        int n = h.length;
+        int[] left = new int[n];
+        left[0] = -1;
+        
+        int[] right = new int[n];
+        right[n-1] = n;
+        
+        //Fill left array
+        for(int i = 1; i < n; i++) {
+            int prev = i-1;
+            while(prev >= 0 && h[prev] >= h[i]){
+                prev = left[prev];
+            }
+            left[i] = prev;
+        }
+        
+        //Fill right array
+        for(int i = n-2; i >= 0; i--){
+            int prev = i+1;
+            while(prev < n && h[prev] >= h[i]){
+                prev = right[prev];
+            }
+            right[i] = prev;
+        }
+        
+        int maxArea = 0;
+        for(int i = 0; i < n; i++) {
+            maxArea = Math.max(maxArea, h[i] * (right[i]-left[i]-1));
+        }
+        
+        return maxArea;
+    }
+}
+
+class Solution {
+    public int largestRectangleArea(int[] h) {
+        //Check null condition
+        if(h == null || h.length == 0) return 0;
+        
+        //Initialize variables left and right
+        int n = h.length;
+        Stack<Integer> stack = new Stack<>();
+        int maxArea = 0;
+        
+        for(int i = 0; i <= n; i++) {
+            int ht = (i == n) ? 0 : h[i];
+            //If stack is empty of ht >= h[top] push in stack
+            //Else process the elements and find area.
+            
+            if(stack.isEmpty() || ht >= h[stack.peek()]) {
+                stack.push(i);
+            }
+            else {
+                //Process elements and find the mexArea for popped index
+                int top = stack.pop();
+                int width = (stack.isEmpty()) ? i : (i - stack.peek() - 1);
+                int area = h[top] * width;
+                maxArea = Math.max(maxArea, area);
+                i--;
+            }
+        }
+        return maxArea;
+    }
+}