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

题目描述

617 Merge Two Binary Trees
https://leetcode-cn.com/problems/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.

Example 1:

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

Note: The merging process must start from the root nodes of both trees.

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解题过程

合并二叉树，递归做，很简单.
时间复杂度 O(n)，空间复杂度 O(n)

private static class SolutionV2020 {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (null == t1 && null == t2) {
            return null;
        }
        if (null == t1 || null == t2) {
            return null == t1 ? t2 : t1;
        }
        // t1,t2都非空时，使用t1作为新的根节点
        t1.val += t2.val;
        t1.left = mergeTrees(t1.left, t2.left);
        t1.right = mergeTrees(t1.right, t2.right);
        return t1;
    }
}

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GitHub代码

algorithms/leetcode/leetcode/_617_MergeTwoBinaryTrees.java
https://github.com/masikkk/algorithms/blob/master/leetcode/leetcode/_617_MergeTwoBinaryTrees.java

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