Binary Tree Paths leetcode

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"]

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);
    }
}