leetcode學習篇六——Binary Tree Paths

題目：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”]

這道題目比較簡單，利用深度優先搜索遍歷二叉樹即可，算法複雜度O(n)，代碼如下：

/**
 * 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> res;
    void search(TreeNode *root, vector<string>& res, string s) {
        if(root->left == NULL && root->right == NULL) {
            res.push_back(s);
            return;
        }
        if(root->left != NULL) search(root->left, res, s+"->"+to_string(root->left->val));
        if(root->right != NULL) search(root->right, res, s+"->"+to_string(root->right->val));
    }
    vector<string> binaryTreePaths(TreeNode* root) {
        if(root == NULL) return res;
        search(root, res, to_string(root->val));
        return res;
    }
};