[LeetCode]257. Binary Tree Paths

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Problem

Given a binary tree, return all root-to-leaf paths.

Note: A leaf is a node with no children. 

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Example:

Input:

   1
 /   \
2     3
 \
  5

Output: ["1->2->5", "1->3"]

Explanation: All root-to-leaf paths are: 1->2->5, 1->3

给定一颗二叉树,返回所有从根节点到叶子节点的路径.

Solution

Analysis

这道题本质上是在考二叉树的前序遍历.其他字符串处理是小枝节. 二叉树的前序遍历:根左右. 最常用的方法递归. 这里需要对遍历结果进行处理,当遍历到叶子节点时,说明找到一条路径,将这条路径保存到结果数组中;其他情况,继续遍历.

处理”->”, 将最后的叶子节点单独处理, 其他情况遍历完,同时加上”->”.

Code

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/**
 * 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> result;
        if (!root) return result;
        string path = "";
        
        preorder(root, path, result);
        
        return result;
    }
private:
    void preorder(TreeNode* root, string path, vector<string>& result){
        if (!root->left && !root->right){
            path += to_string(root->val);
            result.push_back(path);
            return;
        }
        path += to_string(root->val) + "->";
        if (root->left) preorder(root->left, path, result);
        if (root->right) preorder(root->right, path, result);
    }
};
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