leetcode-617-easy

Merge Two Binary Trees

You are given two binary trees root1 and root2.

Imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not. You need to merge the two trees into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of the new tree.

Return the merged tree.

Note: The merging process must start from the root nodes of both trees.

Example 1:

Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7]
Example 2:

Input: root1 = [1], root2 = [1,2]
Output: [2,2]
Constraints:

The number of nodes in both trees is in the range [0, 2000].
-104 <= Node.val <= 104

思路一：递归实现合并二叉树，一开始的思路想复杂了，用递归只要考虑当下节点的操作，其余的处理递归框架会自动实现

    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null) {
            return null;
        } else if (root1 == null) {
            return root2;
        } else if (root2 == null) {
            return root1;
        }

        TreeNode node = new TreeNode(root1.val + root2.val);
        node.left = mergeTrees(root1.left, root2.left);
        node.right = mergeTrees(root1.right, root2.right);

        return node;
    }