思路:
层次化遍历二叉树,但要控制每次进入每层节点的顺序,通过栈和层数来控制。
时间复杂度O(N),空间复杂度O(N)。
/**
* 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 == nullptr) return ans;
stack<TreeNode*> currentLevel, nextLevel;
TreeNode *p = root;
currentLevel.push(p);
vector<int> levelArray;
int levelFlag = 0;//even : left to right , odd : right to left
while(!currentLevel.empty()) {
while(!currentLevel.empty()) {
p = currentLevel.top();
currentLevel.pop();
levelArray.push_back(p->val);
if(levelFlag & 1) {//odd
if(p->right)
nextLevel.push(p->right);
if(p->left)
nextLevel.push(p->left);
}else {
if(p->left)
nextLevel.push(p->left);
if(p->right)
nextLevel.push(p->right);
}
}
ans.push_back(levelArray);
levelArray.clear();
swap(nextLevel, currentLevel);
levelFlag++;
}
return ans;
}
};