leetcod617_合并二叉树

几乎是最简单的递归题了。

class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null && root2 == null) return null;
        else if(root1 != null && root2 != null) {
            TreeNode root = new TreeNode(root1.val + root2.val);
            root.left = mergeTrees(root1.left, root2.left);
            root.right = mergeTrees(root1.right, root2.right);
            return root;
        }
        else {
            if(root1 != null) {
                TreeNode root = new TreeNode(root1.val);
                root.left = mergeTrees(root1.left, null);
                root.right = mergeTrees(root1.right, null);
                return root;
            }
            else {
                TreeNode root = new TreeNode(root2.val);
                root.left = mergeTrees(null, root2.left);
                root.right = mergeTrees(null, root2.right);
                return root;
            }
        }
    }
}

不过如果抓住本质的话，是这样：

class Solution {
    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
        if(root1 == null) return root2;
        if(root2 == null) return root1;

        TreeNode root = new TreeNode(root1.val + root2.val);
        root.left = mergeTrees(root1.left, root2.left);
        root.right = mergeTrees(root1.right, root2.right);
        return root;
    }
}