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"]

用backtracking的方法。与113题很类似。需要注意其实root元素

public IList<string> BinaryTreePaths(TreeNode root) {
        IList<string> res = new List<string>();
        BackTracking(root, "",res,root);
        return res;
    }

    public void BackTracking(TreeNode root, string s, IList<string> res,TreeNode head)
    {
        if(root != null)
        {
            if(root == head) s += root.val;
            else s += "->" + root.val;
            if(root.left == null && root.right == null)
            {
                res.Add(s);
            }
            else
            {
                BackTracking(root.left,s,res,head);
                BackTracking(root.right,s,res,head);
            }
        }
    }