/** * 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 {    class QueueNode {        TreeNode node;        int depth;        public QueueNode(TreeNode node, int depth) {            this.node = node;            this.depth = depth;        }    }
    public int minDepth(TreeNode root) {        if (root == null) {            return 0;        }
        Queue<QueueNode> queue = new LinkedList<>();        queue.offer(new QueueNode(root, 1));        while (!queue.isEmpty()) {            QueueNode nodeDepth = queue.poll();            TreeNode node = nodeDepth.node;            int depth = nodeDepth.depth;            if (node.left == null && node.right == null) {                return depth;            }            if (node.left != null) {                queue.offer(new QueueNode(node.left, depth + 1));            }            if (node.right != null) {                queue.offer(new QueueNode(node.right, depth + 1));            }        }
        return 0;    }}