题目：Given a binary tree, find its minimum depth.The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
/**
* Definition for binary tree
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
import java.util.*;
public class Solution {
public int run(TreeNode root) {
if(root==null)
return 0;
if(root.left==null&&root.right==null)
return 1;
//方法一：递归
/* if(root.left==null)// 若左子树为空，则返回右子树的最小深度+1
return run(root.right)+1;
if(root.right==null)
return run(root.left)+1;
int l=run(root.left);
int r=run(root.right);
return (l>r)?(r+1):(l+1);
*/
//方法二
Queue<TreeNode> q=new LinkedList<>();
// 思路是层序遍历，找到第一个左右结点都为null的情况，就返回
q.add(root);
int level=0;
while(!q.isEmpty()){
int size=q.size();
level++;
for(int i=0;i<size;i++){
TreeNode node=q.poll();
if(node.left==null&&node.right==null)
return level;
if(node.left!=null)
q.add(node.left);
if(node.right!=null)
q.add(node.right);
}
}
return level;
}
}