[leetcode] Binary Tree Zigzag Level Order Traversal

Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree

{3,9,20,#,#,15,7}

,

3
/ \
9  20
/  \
15   7

return its zigzag level order traversal as:

[
[3],
[20,9],
[15,7]
]

思路：

BFS的基础题目。

题解：

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
vector<vector<int> > res;
vector<int> tmp;
queue<TreeNode *> q;
bool flag = true;
int count = 0, num = 1;
if(root==NULL)
return res;
q.push(root);
while(!q.empty()) {
count = 0;
for(int i=0;i<num;i++) {
root = q.front();
q.pop();
tmp.push_back(root->val);
if(root->left) {
q.push(root->left);
count++;
}
if(root->right) {
q.push(root->right);
count++;
}
}
num = count;
if(!flag)
reverse(tmp.begin(), tmp.end());
flag = !flag;
res.push_back(tmp);
tmp.clear();
}
return res;
}
};

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