1. 题目
Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its level order traversal as:
[
[3],
[9,20],
[15,7]
]
2. 思路
用FIFO队列来构建水平遍历。用NULL来分割不同的层。
3. 代码
/**
* 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>> ret;
if (root == NULL) { return ret; }
vector<int> level;
deque<TreeNode*> d;
d.push_back(root);
d.push_back(NULL);
while (!d.empty()) {
TreeNode* n = d.front();
d.pop_front();
if (n == NULL) {
if (level.size() != 0) {
ret.push_back(level);
level.clear();
}
if (d.empty()) { return ret; }
else { d.push_back(NULL); }
} else {
if (n->left != NULL) {
d.push_back(n->left);
}
if (n->right != NULL) {
d.push_back(n->right);
}
level.push_back(n->val);
}
}
return ret;
}
};
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