首先是一段跑不了的代码

错解

/**
 * 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 TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if (root1 == NULL){
            return root2;
        }
        if (root2 == NULL){
            return root1;
        }
        mergeTrees(TreeNode root1, TreeNode root2);
        mergeTrees(TreeNode root1, TreeNode root2);
        return root1+root2;
    }
}

注意题目要我们的是合并后，所以需要构造一个新的树

中序遍历

/**
 * 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 TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if (root1 == null){
            return root2;
        }
        if (root2 == null){
            return root1;
        }
        TreeNode newtree = new TreeNode(root1.val + root2.val);
        newtree.left = mergeTrees(root1.left, root2.left);
        newtree.right = mergeTrees(root1.right, root2.right);
        return newtree;
    }
}

广度搜索

累了