LeetCode OJ--Binary Tree Zigzag Level Order Traversal *

https://oj.leetcode.com/problems/binary-tree-zigzag-level-order-traversal/

树的层序遍历

使用两个 stack 或者 vector 分别表示 当前层和下一层

/**
 * 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> > ans;
        if(root == NULL)
            return ans;
        
        vector<TreeNode *> parent;
        vector<TreeNode *> son;
        parent.push_back(root);
        
        int level = 0;
        while(parent.empty() == false)
        {
            vector<int> ansPiece;
            
            if(level % 2 == 0) // from left to right
            {
                for(int i = parent.size() - 1; i >= 0; i--)
                {
                    TreeNode *current = parent[i];
                    ansPiece.push_back(current->val); // save the ans
                    
                    if(parent[i]->left)
                        son.push_back(parent[i]->left);
                    if(parent[i]->right)
                        son.push_back(parent[i]->right);
                }
            }
            else // from right to left
            {
                for(int i = parent.size() - 1; i >= 0; i--)
                {
                    TreeNode *current = parent[i];
                    ansPiece.push_back(current->val);
                    
                    if(parent[i]->right)
                        son.push_back(parent[i]->right);
                        
                    if(parent[i]->left)
                        son.push_back(parent[i]->left);
                }
            }
            level++;
            parent = son;
            son.clear();
            ans.push_back(ansPiece);
        }
        return ans;
    }
    
};