Jan 12 - Binary Tree Paths; Tree; Recursion; DFS;

代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> list = new ArrayList<>();
        if(root == null) return list;
        String s = "";
        addTreePath(root, root.val, s, list);
        return list;
    }
    
    public void addTreePath(TreeNode root, int val, String s, List<String> list){
        if(root.left == null && root.right == null){
            list.add(s+root.val);
        }
        else if(root.left != null && root.right == null){
            addTreePath(root.left, val, s+root.val+"->", list);
        }
        else if(root.left == null && root.right != null){
            addTreePath(root.right, val, s+root.val+"->", list);
        }
        else{
            addTreePath(root.left, val, s+root.val+"->", list);
            addTreePath(root.right, val, s+root.val+"->", list);
        }
    }
}