木又连续日更第67天(67/100)
木又的第115篇leetcode解题报告
二叉树
类型第5篇解题报告
leetcode第102题:二叉树的层次遍历
https://leetcode-cn.com/problems/binary-tree-level-order-traversal/
【题目】
给定一个二叉树,返回其按层次遍历的节点值。(即逐层地,从左到右访问所有节点)。
例如:
给定二叉树: [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回其层次遍历结果:
[
[3],
[9,20],
[15,7]
]
【思路】
使用队列,初始化为根节点,接着弹出队列元素,将其left节点和right节点加入到新队列中。原队列为空,而新队列不为空时,将新队列的元素赋值给原队列。
【代码】
python版本
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def levelOrder(self, root):
"""
:type root: TreeNode
:rtype: List[List[int]]
"""
if not root:
return []
res = []
num = []
tmp1 = [root]
tmp2 = []
while len(tmp1) != 0 or len(tmp2) != 0:
# tmp1为空,tmp2不为空
if len(tmp1) == 0:
tmp1 = copy.copy(tmp2)
tmp2 = []
res.append(num)
num = []
p = tmp1.pop(0)
num.append(p.val)
if p.left:
tmp2.append(p.left)
if p.right:
tmp2.append(p.right)
res.append(num)
return res
C++版本
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<vector<int>> levelOrder(TreeNode* root) {
vector<vector<int>> res;
if(!root)
return res;
vector<int> num;
queue<TreeNode*> tmp1;
tmp1.push(root);
queue<TreeNode*> tmp2;
TreeNode* p;
while(!tmp1.empty() || !tmp2.empty()){
// tmp1为空,tmp2不为空,则交换tmp1和tmp2
if(tmp1.empty()){
while(!tmp2.empty()){
tmp1.push(tmp2.front());
tmp2.pop();
}
res.push_back(num);
num.erase(num.begin(), num.end());
}
p = tmp1.front();
tmp1.pop();
num.push_back(p->val);
if(p->left)
tmp2.push(p->left);
if(p->right)
tmp2.push(p->right);
}
res.push_back(num);
return res;
}
};