题解 | 按之字形顺序打印二叉树

使用双端队列遍历，从左向右遍历时从队首读取到队尾，从右向左遍历时从队尾读取到队首。

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param pRoot TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > Print(TreeNode* pRoot) {
        // write code here
        vector<vector<int>> res;
        if (pRoot == nullptr) return res;
        deque<TreeNode*> q;
        q.push_back(pRoot);
        int level = 1;
        while (!q.empty()) {
            vector<int> cur;
            for (int i = q.size(); i > 0; i--) {
                if (level % 2 == 1) {
                    TreeNode* node = q.front(); q.pop_front();
                    cur.push_back(node->val);
                    if (node->left != nullptr) {
                        q.push_back(node->left);
                    }
                    if (node->right != nullptr) {
                        q.push_back(node->right);
                    }
                } else {
                    TreeNode* node = q.back(); q.pop_back();
                    cur.push_back(node->val);
                    if (node->right != nullptr) {
                        q.push_front(node->right);
                    }
                    if (node->left != nullptr) {
                        q.push_front(node->left);
                    }
                }
            }
            res.push_back(cur);
            level++;
        }
        return res;
    }
};