力扣 257 二叉树的所有路径

题意：给定一个树，返回从根到叶节点的所有路径

思路：先序遍历树，获取所有路径

思想：树的先跟遍历

复杂度：时间O（n²)，空间O(n)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<String> res = new ArrayList();
    public List<String> binaryTreePaths(TreeNode root) {
        get(new StringBuilder(), root);
        return res;
    }
    
    void get(StringBuilder temp, TreeNode root) {
        if(root == null) {
            return;
        }
        if(root.left == null && root.right == null) {
            temp.append(root.val);
            res.add(temp.toString());
            return;
        }
        temp.append(root.val).append("->");
        int len = temp.length();
        get(temp, root.left);
        temp.setLength(len);
        get(temp, root.right);
    }
}