leetcode 103. Binary Tree Zigzag Level Order Traversal

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本文介绍了一种算法，用于实现二叉树的锯齿形层序遍历，即从左到右再从右到左交替进行节点值的收集。通过使用两个栈，该算法可以有效地按层级顺序输出节点值，同时保持每层的顺序符合锯齿形要求。

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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],

return its zigzag level order traversal as:

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

/**
 * 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) {
            return result;
        }
        
        stack<TreeNode*> s1;
        stack<TreeNode*> s2;
        
        s1.push(root);
        
        while (!s1.empty() || !s2.empty()) {
            vector<int> row;
            
            if (!s1.empty()) {
                while (s1.size() > 0) {
                    TreeNode* node = s1.top();
                    s1.pop();

                    row.push_back(node->val);

                    if (node->left) {
                        s2.push(node->left);
                    }
                    if (node->right) {
                        s2.push(node->right);
                    }
                }
            }
            else {
                while (s2.size() > 0) {
                    TreeNode* node = s2.top();
                    s2.pop();
                    
                    row.push_back(node->val);
                    
                    if (node->right) {
                        s1.push(node->right);
                    }
                    if (node->left) {
                        s1.push(node->left);
                    }
                }    
            }
            
            result.push_back(row);
        }
        
        return result;
    }
};