LeetCode 617. 合并二叉树(Merge Two Binary Trees)

617. 合并二叉树

617. Merge Two Binary Trees

题目描述

给定两个二叉树，想象当你将它们中的一个覆盖到另一个上时，两个二叉树的一些节点便会重叠。

你需要将他们合并为一个新的二叉树。合并的规则是如果两个节点重叠，那么将他们的值相加作为节点合并后的新值，否则不为 NULL 的节点将直接作为新二叉树的节点。

LeetCode617. Merge Two Binary Trees

示例 1:

输入:
```
Tree 1 Tree 2
1 2
/ \ / \
3 2 1 3
/ \ \
5 4 7
```
输出:
合并后的树:
```
3
/ \
4 5
/ \ \
5 4 7
```

注意: 合并必须从两个树的根节点开始。

Java 实现

TreeNode 类

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }
}

Recursive Solution

class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null) {
            return t2;
        }
        if (t2 == null) {
            return t1;
        }
        TreeNode root = new TreeNode(t1.val + t2.val);
        root.left = mergeTrees(t1.left, t2.left);
        root.right = mergeTrees(t1.right, t2.right);
        return root;
    }
}

class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) {
            return null;
        }
        int val = (t1 == null ? 0 : t1.val) + (t2 == null ? 0 : t2.val);
        TreeNode root = new TreeNode(val);
        root.left = mergeTrees(t1 == null ? null : t1.left, t2 == null ? null : t2.left);
        root.right = mergeTrees(t1 == null ? null : t1.right, t2 == null ? null : t2.right);
        return root;
    }
}

参考资料

• https://leetcode.com/problems/merge-two-binary-trees/
• https://leetcode-cn.com/problems/merge-two-binary-trees/