LeetCode: 103_Binary Tree Zigzag Level Order Traversal | 二叉树Zigzag层次遍历 | Medium

本题也属于层次遍历的变形，不同之处在于其遍历的方法是交替进行的，形成一个ZigZag的曲线形式，如下：

代码如下：

struct TreeNode {
    int            val;
    TreeNode*    left;
    TreeNode*    right;
    TreeNode(int x): val(x), left(NULL),right(NULL) {}
};

void Swap(vector<int> &ivec)
{
    int temp = 0;
    int len = ivec.size();
    for (int i = 0; i < len/2; i ++) {
        temp = ivec.at(i);
        ivec.at(i) = ivec.at(len-1-i);
        ivec.at(len-1-i) = temp;
    }
}

vector<vector <int> > zigzagLevelOrder(TreeNode *root)
{
    vector<vector<int> > tree_vector;
    vector<int> ivec;
    queue<TreeNode *> tree_queue;
    if (NULL == root)
        return tree_vector;

    tree_queue.push(root);
    tree_queue.push(NULL);
    int nLevelCount = 1;
    while (true) {
        TreeNode *pTemp = tree_queue.front();
        tree_queue.pop();
        if (pTemp == NULL) {
            if (nLevelCount%2 == 0) { //if the num of level is odd, swap the ivec;
                Swap(ivec);
            }
            tree_vector.push_back(ivec);
            ivec.clear();
            if (tree_queue.empty())
                break;
            tree_queue.push(NULL);
            nLevelCount ++;
        }
        else {
            ivec.push_back(pTemp->val);
            if (pTemp->left)
                tree_queue.push(pTemp->left);
            if (pTemp->right)
                tree_queue.push(pTemp->right);
        }
    }
    return tree_vector;
}