257. Binary Tree Paths

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

For example, given the following binary tree:

   1
 /   \
2     3
 \
  5

All root-to-leaf paths are:

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

Tree类题目。

解题思路为DFS。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> res = new ArrayList<String>();
        helper(root, res, "");
        return res;
    }
    
    private void helper(TreeNode node, List<String> res, String path) {
        if(node == null) return;
        if(node.left == null && node.right == null) {
            path += node.val;
            res.add(path);
            return;
        }
        path = path + node.val + "->";
        helper(node.left, res, path);
        helper(node.right, res, path);
    }
}