0617.md

617. Merge Two Binary Trees

Level:

Topic:

Question

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 the two trees 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 the new tree.

Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7] \

Intuition

Recursive:

• combine the the value of root1 and root2
• if either current node is null, return the other
• assign root1's children as recursive calls
• return root1

Code

Time: O(n)
Space: O(n)

Recursive

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

    if (root2 == null)
        return root1;

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