1126. Merge Two Binary Trees

Given two binary trees and 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 them 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 new tree.

Input:
Tree 1 Tree 2
1 2
/ \ / \
3 2 1 3
/ \ \
5 4 7
Output:
Merged tree:
3
/ \
4 5
/ \ \
5 4 7

public class Solution {
    /**
     * @param t1: the root of the first tree
     * @param t2: the root of the second tree
     * @return: the new binary tree after merge
     */
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        // Write your code here
        if (t1 == null && t2 == null) {
            return null;
        }

        if (t1 == null && t2 != null) {
            return t2;
        }

        if (t1 != null && t2 == null) {
            return t1;
        }

        TreeNode left = mergeTrees(t1.left, t2.left);
        TreeNode right = mergeTrees(t1.right, t2.right);

        TreeNode node = new TreeNode(t1.val + t2.val);
        node.left = left;
        node.right = right;

        return node;
    }
}