c++ 之字形打印树

class Solution {
public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        vector<vector<int>> res;
        if (pRoot == nullptr) return res;

        stack<TreeNode*> left;
        stack<TreeNode*> right;
        left.push(pRoot);
        int flag = 0;//flag为0 左栈出，进右栈
        while (!left.empty() || !right.empty()){
            vector<int> temp;

            if (flag == 0){
                auto length = left.size();
                for (decltype(length) i = 0; i != length; ++i){
                    if (left.top() != nullptr){
                        temp.push_back(left.top()->val); //vec记录
                        right.push(left.top()->left);
                        right.push(left.top()->right); //子树进右栈
                    }
                    left.pop(); //清空栈顶 
                }
                flag = 1;
            }

            else if (flag == 1){
                auto length = right.size();
                for (decltype(length) i = 0; i != length; ++i){
                    if (right.top() != nullptr){
                        temp.push_back(right.top()->val); //vec记录
                        left.push(right.top()->right);
                        left.push(right.top()->left); //子树进左栈
                    }
                    right.pop(); //清空栈顶
                }
                flag = 0;
            }
            if (!temp.empty())
                res.push_back(temp);
        }
        return res;
    }
};

题目说之字形按层打印树，故考虑广度优先搜索。
1、因为要之字形打印，常规队列存储无法满足需求，采用两个栈（左栈和右栈），轮换存储的方式；
2、存储规则为左栈进栈时先右子后左子，右栈相反；初始化root放在左栈
欢迎交流指正！！！