题解 | #牛群的最短路径#

import java.util.*;

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

public class Solution {
    /**
     * 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 minDepth (TreeNode root) {
        // write code here
        if (root == null) return 0;
        minDepth = Integer.MAX_VALUE;
        levelOrder(root, 0);
        return minDepth;
    }
    private int minDepth;
    private void levelOrder(TreeNode root, int level) {
        if (root == null) return;
        level += 1;
        if (root.left == null && root.right == null) {
            minDepth = Math.min(level, minDepth);
        }
        levelOrder(root.left, level);
        levelOrder(root.right, level);
    }
}

• 边界值处理
• 基于二叉树的层序遍历，查找到叶子节点，更新最短路径值