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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
    public int num;
    public String operand;
    public Node left;
    public Node right;
};

/**
 * 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 TreeNode extends Node {

    public int evaluate() {
        if (this.operand == null) {
            return this.num;
        }
        int leftTreeVal = this.left.evaluate();
        int rightTreeVal = this.right.evaluate();
        int ans = compute(leftTreeVal, rightTreeVal, this.operand);
        return ans;
    }

    public TreeNode(int num) {
        this.num = num;
    }

    public TreeNode(String operand) {
        this.operand = operand;
    }

    public int compute(int i, int j, String operand) {
        int ans = 0;
        switch (operand) {
            case "+":
                ans = i + j;
                break;
            case "-":
                ans = i - j;
                break;
            case "*":
                ans = i * j;
                break;
            case "/":
                ans = i / j;
                break;
        }
        return ans;
    }
}

class TreeBuilder {
    Node buildTree(String[] postfix) {
        Deque<Node> stack = new ArrayDeque<>();
        for (String s : postfix) {
            if (!isOperand(s)) {
                stack.push(new TreeNode(Integer.parseInt(s)));
            } else {
                Node rightNode = stack.pop();
                Node leftNode = stack.pop();
                Node currNode = new TreeNode(s);
                currNode.left = leftNode;
                currNode.right = rightNode;
                stack.push(currNode);
            }
        }
        return stack.pop();
    }

    public boolean isOperand(String s) {
        return s.equals("+") || s.equals("-") || s.equals("*") || s.equals("/");
    }
};

/**
 * Your TreeBuilder object will be instantiated and called as such:
 * TreeBuilder obj = new TreeBuilder();
 * Node expTree = obj.buildTree(postfix);
 * int ans = expTree.evaluate();
 */

这道题还挺好玩的. OOP.

时间复杂度和空间复杂度不是重点.