/*
 * @lc app=leetcode.cn id=111 lang=java
 *
 * [111] 二叉树的最小深度
 *
 * https://leetcode-cn.com/problems/minimum-depth-of-binary-tree/description/
 *
 * algorithms
 * Easy (42.41%)
 * Likes:    277
 * Dislikes: 0
 * Total Accepted:    85.7K
 * Total Submissions: 200.5K
 * Testcase Example:  '[3,9,20,null,null,15,7]'
 *
 * 给定一个二叉树，找出其最小深度。
 *
 * 最小深度是从根节点到最近叶子节点的最短路径上的节点数量。
 *
 * 说明: 叶子节点是指没有子节点的节点。
 *
 * 示例:
 *
 * 给定二叉树 [3,9,20,null,null,15,7],
 *
 * ⁠   3
 * ⁠  / \
 * ⁠ 9  20
 * ⁠   /  \
 * ⁠  15   7
 *
 * 返回它的最小深度  2.
 *
 */

// @lc code=start
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public int minDepth(TreeNode root) {
    if (root == null) {
      return 0;
    }

    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    int depth = 1;

    while (!queue.isEmpty()) {
      int size = queue.size();
      for (int i = 0; i < size; i++) {
        TreeNode node = queue.poll();
        if (node.left == null && node.right == null) {
          return depth;
        }
        if (node.left != null) {
          queue.offer(node.left);
        }
        if (node.right != null) {
          queue.offer(node.right);
        }
      }
      depth++;
    }

    return depth;
  }
}
// @lc code=end

