Minimum Depth of Binary Tree

Easy

Given the root of a binary tree, return its minimum depth — the number of nodes along the shortest path from the root to the nearest leaf (a node with no children).

Example 1

Input:  root = [3,9,20,null,null,15,7]
Output: 2

Example 2

Input:  root = [2,null,3,null,4,null,5,null,6]
Output: 5

Constraints

• 0 <= number of nodes <= 10^5
• -1000 <= Node.val <= 1000