Create Design an Expression Tree With Evaluate Function.java

Author: varunu28

Medium/Design an Expression Tree With Evaluate Function.java

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+/**
+ * This is the interface for the expression tree Node.
+ * You should not remove it, and you can define some classes to implement it.
+ */
+
+abstract class Node {
+  public abstract int evaluate();
+  // define your fields here
+};
+
+class NumericNode extends Node {
+  
+  private int val;
+  
+  public NumericNode(int val) {
+    this.val = val;  
+  }
+  
+  public int evaluate() {
+    return this.val;
+  }
+}
+
+class OperatorNode extends Node {
+  private char operator;
+  private Node leftNode;
+  private Node rightNode;
+  
+  public OperatorNode(char operator, Node leftNode, Node rightNode) {
+    this.operator = operator;
+    this.leftNode = leftNode;
+    this.rightNode = rightNode;
+  }
+  
+  public int evaluate() {
+    int leftValue = this.leftNode == null ? 0 : this.leftNode.evaluate();
+    int rightValue = this.rightNode == null ? 0 : this.rightNode.evaluate();
+    if (this.operator == '+') {
+      return leftValue + rightValue;
+    } else if (this.operator == '-') {
+      return leftValue - rightValue;
+    } else if (this.operator == '*') {
+      return leftValue * rightValue;
+    } else {
+      return leftValue / rightValue;
+    }
+  }
+}
+
+
+/**
+ * This is the TreeBuilder class.
+ * You can treat it as the driver code that takes the postinfix input 
+ * and returns the expression tree represnting it as a Node.
+ */
+
+class TreeBuilder {
+  Node buildTree(String[] postfix) {
+    Stack<Node> stack = new Stack<>();
+    for(String token: postfix){
+      if (token.equals("*") || token.equals("+") || token.equals("-") || token.equals("/")) {
+        Node o2 = stack.pop();
+        Node o1 = stack.pop();
+        stack.push(new OperatorNode(token.charAt(0), o1, o2));
+      } else{
+        stack.push(new NumericNode(Integer.parseInt(token)));
+      }
+    }
+    return stack.pop();
+  }
+};
+
+
+/**
+ * Your TreeBuilder object will be instantiated and called as such:
+ * TreeBuilder obj = new TreeBuilder();
+ * Node expTree = obj.buildTree(postfix);
+ * int ans = expTree.evaluate();
+ */