二叉树根到叶子所有数字的和

图示

代码

import java.util.ArrayList;
import java.util.List;

class Node {
    int val;
    Node left;
    Node right;

    public Node(int val) {
        this.val = val;
    }
}

public class Main {
    private static List<Integer> list;

    public static void main(String[] args) {
        // 叶子节点
        Node n1 = new Node(1);
        Node n2 = new Node(5);
        Node n3 = new Node(1);
        // 根节点
        Node root = new Node(1);
        // 其他节点
        Node o1 = new Node(2);
        Node o2 = new Node(2);
        // 组装
        root.left = o1;
        o1.left = n1;
        o1.right = n2;
        root.right = o2;
        o2.right = n3;
        //  计算结果
        list = new ArrayList<>();
        int result = findBinaryTreeSum(root);
        System.out.println(result);
    }

    private static int findBinaryTreeSum(Node root) {
        if (root == null) return 0;
        list.add(root.val);
        if (root.left == null && root.right == null) { // 到达叶子节点，开始计算
            int num = sumList(list);
            list.remove(list.size() - 1);
            return num;
        } else {
            int subSum = findBinaryTreeSum(root.left) + findBinaryTreeSum(root.right);
            list.remove(list.size() - 1);
            return subSum;
        }
    }

    private static int sumList(List<Integer> list) {
        int num = 0;
        for (Integer integer : list) {
            num = num * 10 + integer;
        }
        return num;
    }
}