257.二叉树的所有路径

方法：深度优先搜索

/**
 * 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 {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> ans = new ArrayList<>();
        StringBuilder sb = new StringBuilder();
        dfs(root,ans,sb);
        return ans;

    }
    public void dfs(TreeNode root,List<String> paths,StringBuilder s){
        if(root == null){
            return;
        } 
        s.append(root.val);
        if(root.left == null && root.right == null){
            paths.add(s.toString());
            return;
        }
        dfs(root.left,paths,new StringBuilder(s).append("->"));
        dfs(root.right,paths,new StringBuilder(s).append("->"));
        
    }
}