LeetCode111. 二叉树的最小深度

输入：root = [3,9,20,null,null,15,7]
输出：2

输入：root = [2,null,3,null,4,null,5,null,6]
输出：5

class Solution {
        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;
            }
        }

        public int minDepth(TreeNode root) {
            if (root == null) {
                return 0;
            }
            Queue<TreeNode> treeNodes = new LinkedList<>();
            treeNodes.add(root);
            int step = 1;
            while (!treeNodes.isEmpty()) {
                int size = treeNodes.size();
                for (int i = 0; i < size; i++) {
                    TreeNode node = treeNodes.poll();
                    if (node.left == null && node.right == null) {
                        return step;
                    }
                    if (node.left != null) {
                        treeNodes.add(node.left);
                    }
                    if (node.right != null) {
                        treeNodes.add(node.right);
                    }
                }
                step++;
            }
            return step;
        }
    }