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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);            }        }    }

 

257. Binary Tree Paths