p113.java

// https://leetcode.com/problems/binary-tree-preorder-traversal/
/*Given the root of a binary tree, return the preorder traversal of its nodes' values.

 

Example 1:


Input: root = [1,null,2,3]
Output: [1,2,3]
Example 2:

Input: root = []
Output: []
Example 3:

Input: root = [1]
Output: [1]
 

Constraints:

The number of nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100*/

/**
 * 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<Integer> preorderTraversal(TreeNode root) {
        List<Integer> ls=new ArrayList<Integer>();
        if(root==null){
            return ls;
        }else{
            ls.add(root.val);
            ls.addAll(preorderTraversal(root.left));
            ls.addAll(preorderTraversal(root.right));
        }
        return ls;
    }
}