LeetCode-M-Binary Tree Zigzag Level Order Traversal

題意

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]
]

解法

堆棧

實現

/**
 * 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>> result;
        if(root == NULL) return result;
        stack<TreeNode*> left;
        stack<TreeNode*> right;
        right.push(root);
        while(!right.empty() || !left.empty()){
            vector<int> sr;
            while(!right.empty()){
                TreeNode* node = right.top();
                right.pop();
                sr.push_back(node->val);
                if(node->left != NULL) left.push(node->left);
                if(node->right != NULL) left.push(node->right);
            }
            if(sr.size() != 0) result.push_back(sr);
            vector<int> sl;
            while(!left.empty()){
                TreeNode* node = left.top();
                left.pop();
                sl.push_back(node->val);
                if(node->right != NULL) right.push(node->right);
                if(node->left != NULL) right.push(node->left);
            }
            if(sl.size() != 0) result.push_back(sl);
        }
        return result;
    }
};