257. Binary Tree Paths

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"]

Solution：DFS

Time Complexity: O(N) Space Complexity: O(N) 递归缓存

Solution Code：

class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> result = new ArrayList<>();
        dfs(root, "", result);
        return result;
    }
    
    private void dfs(TreeNode node, String cur_res, List<String> result) {
        if(node == null) return;
        if(node.left == null && node.right == null) {
            result.add(cur_res + String.valueOf(node.val));
            return;
        }
        cur_res += String.valueOf(node.val) + "->";
        dfs(node.left, cur_res, result);
        dfs(node.right, cur_res, result);
    } 
}