Binary Tree Paths

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

For example, given the following binary tree:   1 /   \2     3 \  5All root-to-leaf paths are:["1->2->5", "1->3"]

深度优先搜索

复杂度

时间O(n) 空间栈O(n logn)

代码

public List<String> binaryTreePaths(TreeNode root) {    List<String> res = new ArrayList<String>();    if (root == null) {        return res;    }    String str = root.val +"";    helper(res, root, str);    return res;}public void helper(List<String> res, TreeNode root, String str) {    if (root.left == null && root.right == null) {        res.add(str);        return;    }    if (root.left != null) {        helper(res, root.left, str + "->" + root.left.val);    }    if (root.right != null) {        helper(res, root.right, str +"->" + root.right.val);    }}