原题链接: https://oj.leetcode.com/problems/minimum-depth-of-binary-tree/

public int minDepth(TreeNode root) {
    if (root == null) return 0;

    if (root.left == null && root.right == null) {
        return 1;
    }  else if (root.left != null && root.right != null) {
        return Math.min(minDepth(root.left) + 1, minDepth(root.right) + 1);
    } else if (root.left != null) {
        return minDepth(root.left) + 1;
    } else {
        return minDepth(root.right) + 1;
    }
}