https://leetcode.com/problems/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,null,null,15,7],
3
/
9 20
/
15 7
return its zigzag level order traversal as:
[ [3], [20,9], [15,7] ]
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>> zigzagLevelOrder(TreeNode* root) {
vector<vector<int> > ans;
if(!root) return ans;
level(root, ans, 1);
for(int i = 0; i < ans.size(); i ++) {
if(i % 2) {
for(int j = 0; j < ans[i].size() / 2; j ++)
swap(ans[i][j], ans[i][ans[i].size() - j - 1]);
}
}
return ans;
}
void level(TreeNode* root, vector<vector<int> >& ans, int depth) {
vector<int> v;
if(depth > ans.size()) {
ans.push_back(v);
v.clear();
}
ans[depth - 1].push_back(root -> val);
if(root -> left)
level(root -> left, ans, depth + 1);
if(root -> right)
level(root -> right, ans, depth + 1);
}
};