Merge Two Binary Trees
Problem
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:
Input:
Tree 1 Tree 2
1 2
/ \ / \
3 2 1 3
/ \ \
5 4 7
Output:
Merged tree:
3
/ \
4 5
/ \ \
5 4 7
Solution
public class Solution {
public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
if (t1 == null) return t2;
if (t2 == null) return t1;
TreeNode node = new TreeNode(t1.val + t2.val);
node.left = mergeTrees(t1.left, t2.left);
node.right = mergeTrees(t1.right, t2.right);
return node;
}
}
Analysis
This is a typical traversal problem and we use typical recursive solution
Notice that we need to return an object of TreeNode
hence we cannot return the sum of values in each recursion
In this solution, we construct a new TreeNode
using the sum of two given input TreeNodes
And then we recursively set the left of newly constructed TreeNode
and also the right
At the end, we return our newNode
because it's the root in the first recursion call