class Solution {
    public int minDepth(TreeNode root) {

        if (root == null){
            return 0;
        }

        int left = minDepth(root.left);
        int right = minDepth(root.right);

        /**
         * 如果左右孩子有一个没有，那就只计算另外一个子树
         */
        if (left == 0) {
            return right + 1;
        }

        if (right == 0) {
            return left + 1;
        }

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

https://leetcode-cn.com/problems/minimum-depth-of-binary-tree/