Leetcode480-Binary Tree Paths-Easy

480. Binary Tree Paths

Given a binary tree, return all root-to-leaf paths.

Example

Example 1:

Input:

   1
 /   \
2     3
 \
  5

Output:

[
  "1->2->5",
  "1->3"
]

Example 2:

Input:

   1
 /
2     

Output:

[
  "1->2"
]

注意：

因为题目的output格式 "1 -> 2"，（有箭头），所以用String来存储每一个path。

递归法代码：

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: the root of the binary tree
     * @return: all root-to-leaf paths
     */
    ArrayList<String> result;

    public List<String> binaryTreePaths(TreeNode root) {
        result = new ArrayList<String>();
        if (root == null) {
            return result;
        }

        String path = String.valueOf(root.val);
        helper(root, path);
        return result;
    }
    public void helper(TreeNode root, String path) {

        if (root.left == null && root.right == null) {
            result.add(path);
            return;
        }

        if (root.left != null) {
            helper(root.left, path + "->" + String.valueOf(root.left.val));
        }

        if (root.right != null) {
            helper(root.right, path + "->" + String.valueOf(root.right.val));
        }
    }
}