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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private TreeNode helper(TreeNode t1, TreeNode t2) {
        if (t2 == null) return t1;
        if (t1 == null) return t2;
        t1.val += t2.val;
        t1.left = helper(t1.left, t2.left);
        t1.right = helper(t1.right, t2.right);
        return t1;
    }
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        return helper(t1, t2);
    }
}

/*
[1,3,2,5]
[2,1,3,null,4,null,7]
[1,2,3]
[1,2,3,4,5,6,7]

[1,2,null,3]
[1,null,2,null,3]
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if(t1 == null) return t2;
        if(t2 == null) return t1;
        
        TreeNode left = mergeTrees(t1.left, t2.left);
        TreeNode right = mergeTrees(t1.right, t2.right);
        
        t1.val += t2.val;
        t1.left = left;
        t1.right = right;
        return t1;
    }
}