[111. Minimum Depth of Binary Tree](https://leetcode.com/problems/minimum-depth-of-binary-tree/)

### 題目描述

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

**Note:** A leaf is a node with no children.

### 範例

**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**:

* The number of nodes in the tree is in the range [0, 10^5^].
* -1000 <= `Node.val` <= 1000

### 解答
#### Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int minDepth(TreeNode root) {
        if (root == null) 
            return 0;
            
        if(root.left == null)
            return minDepth(root.right) + 1;
        else if(root.right == null)
            return minDepth(root.left) + 1;

        return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
    }
}

> [name=Ron Chen][time=Mon, Jul 10, 2023]

### Reference

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