617. 合并二叉树

递归

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

    public TreeNode merge(TreeNode res,TreeNode t1,TreeNode t2) {
        if(t1 == null && t2 == null) {
            return null;
        }
        if(t1 != null && t2 != null) {
            t1.val += t2.val;
        }else if(t2 != null) {
            t1 = new TreeNode(0);
            t1.val = t2.val;
        } else if (t1 != null){
            t2 = new TreeNode(0);
        }
        res = new TreeNode(t1.val);
        res.left = merge(res.left,t1.left,t2.left);
        res.right = merge(res.right,t1.right,t2.right);
        return res;
    }
}