方法1：DFS (递归)

时间复杂度：$O(n)$

空间复杂度：$O(H)$

解题思路

https://leetcode.cn/problems/minimum-depth-of-binary-tree/solution/er-cha-shu-de-zui-xiao-shen-du-by-leetcode-solutio/

Java 代码

/**
 * 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) {
        int min_depth = Integer.MAX_VALUE;
        if (root == null) {
            return 0;
        }
        if (root.left == null && root.right == null) {
            return 1;
        }
        if (root.left != null) {
            min_depth = Math.min(min_depth, minDepth(root.left));
        }
        if (root.right != null) {
            min_depth = Math.min(min_depth, minDepth(root.right));
        }
        return min_depth + 1;
    }
}