题解 | #二叉树的最小深度#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */


/**
 * NC234 二叉树的最小深度
 * @author d3y1
 */
public class Solution {
    private int result = Integer.MAX_VALUE;

    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param root TreeNode类
     * @return int整型
     */
    public int run (TreeNode root) {
        return solution1(root);
        // return solution2(root);
    }

    /**
     * bfs
     * @param root
     * @return
     */
    private int solution1(TreeNode root){
        if(root == null){
            return 0;
        }

        levelOrder(root);

        return result;
    }

    /**
     * 层序遍历: 队列
     * @param root
     */
    private void levelOrder(TreeNode root){
        if(root == null){
            return;
        }

        Queue<QueueNode> queue = new LinkedList<>();
        queue.offer(new QueueNode(root, 1));
        QueueNode queueNode;
        while(!queue.isEmpty()){
            queueNode = queue.poll();
            if(queueNode.node != null){
                // 叶子节点
                if(queueNode.node.left==null && queueNode.node.right==null){
                    result = queueNode.depth;
                    break;
                }
                if(queueNode.node.left != null){
                    queue.offer(new QueueNode(queueNode.node.left, queueNode.depth+1));
                }
                if(queueNode.node.right != null){
                    queue.offer(new QueueNode(queueNode.node.right, queueNode.depth+1));
                }
            }
        }
    }

    /**
     * dfs
     * @param root
     * @return
     */
    private int solution2(TreeNode root){
        if(root == null){
            return 0;
        }

        postOrder(root, 1);

        return result;
    }

    /**
     * 后序遍历: 递归
     * @param root
     * @param depth
     */
    private void postOrder(TreeNode root, int depth){
        if(root == null){
            return;
        }

        if(depth < result){
            postOrder(root.left, depth+1);
            postOrder(root.right, depth+1);
            // 当前节点叶子节点
            if(root.left==null && root.right==null){
                result = depth;
            }
        }
    }

    /**
     * 队列节点
     */
    private class QueueNode {
        TreeNode node;
        int depth;

        public QueueNode(TreeNode node, int depth){
            this.node = node;
            this.depth = depth;
        }
    }
}