103-binary-tree-zigzag-level-order-traversal.md

103. Binary Tree Zigzag Level Order Traversal - 二叉树的锯齿形层次遍历

给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回锯齿形层次遍历如下：

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

---

题目标签：Stack / Tree / Breadth-first Search

题目链接：LeetCode / LeetCode中国

题解

Language	Runtime	Memory
cpp	4 ms	942.1 KB

/**
 * 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> > res;
        if (!root) { return res; }
        queue<TreeNode*> Q;
        Q.push(root);
        while (!Q.empty()) {
            int n = Q.size();
            res.push_back(vector<int>());
            while (n--) {
                TreeNode* t = Q.front();
                Q.pop();
                res.back().push_back(t->val);
                if (t->left) {
                    Q.push(t->left);
                }
                if (t->right) {
                    Q.push(t->right);
                }
            }
            // reverse even layers
            if (res.size() % 2 == 0) {
                reverse(res.back().begin(), res.back().end());
            }
        }
        return res;
    }
};