257 Binary Tree Paths 二叉树的所有路径

给定一个二叉树，返回从根节点到叶节点的所有路径。
例如，给定以下二叉树:
   1
 /   \
2     3
 \
  5
所有根到叶路径是:
["1->2->5", "1->3"]

详见：https://leetcode.com/problems/binary-tree-paths/description/

Java实现：

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

C++实现：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<string> binaryTreePaths(TreeNode* root) {
        vector<string> res;
        if(root)
        {
            helper(root,"",res);
        }
        return res;
    }
    void helper(TreeNode *root,string out,vector<string> &res)
    {
        out+=to_string(root->val);
        if(!root->left&&!root->right)
        {
            res.push_back(out);
        }
        if(root->left)
        {
            helper(root->left,out+"->",res);
        }
        if(root->right)
        {
            helper(root->right,out+"->",res);
        }
    }
};

参考：https://www.cnblogs.com/grandyang/p/4738031.html