leetCode-257:Binary Tree Paths

问题描述

给定一个二叉树，要求输出所有二叉树的路径（从根节点到叶子节点的路径），题目链接：点我

样例输入输出

输入：root = [1,2,3,null,5]

输出：[“1->2->5”,”1->3”]

解释：输入的二叉树如下

输入：root = [1]

输出：[1]

问题解法

使用先序遍历方式，将每个节点添加到路径上。代码如下

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        if (root.left == null && root.right == null) {
            List<String> result = new ArrayList<>();
            result.add(String.valueOf(root.val));
            return result;
        }

        List<String> result = new ArrayList<>();
        if (root.left != null) {
            List<String> left = binaryTreePaths(root.left);
            merge(result, left, String.valueOf(root.val));
        }

        if (root.right != null) {
            List<String> right = binaryTreePaths(root.right);
            merge(result, right, String.valueOf(root.val));
        }

        return result;
    }

    private void merge(List<String> result, List<String> origin, String value) {
        for (String item : origin) {
            result.add(value + "->" + item);
        }
    }
}