题解 | #合并二叉树# 先序遍历

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param t1 TreeNode类 
     * @param t2 TreeNode类 
     * @return TreeNode类
     */
    public TreeNode mergeTrees (TreeNode t1, TreeNode t2) {
        // write code here
        if (t1 == null) {
            return t2;
        }
        if (t2 == null) {
            return t1;
        }
        preorder(t1, t2);
        return t1;
    }

    private void preorder(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) {
            return;
        }
        t1.val += t2.val;
        if (t1.left == null) {
            t1.left = t2.left;
        } else if (t2.left != null){
            preorder(t1.left, t2.left);
        }
        if (t1.right == null) {
            t1.right = t2.right;
        } else if (t2.right != null) {
            preorder(t1.right, t2.right);
        }
    }
}