题解 | #按之字形顺序打印二叉树#

双端队列就相当于双栈：

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
  public:
    vector<vector<int> > Print(TreeNode* pRoot) {
        vector<vector<int> > ret;
        if (pRoot == nullptr) return ret;

        deque<TreeNode*> s;
        s.push_back(pRoot);
        int level = 0;
        while (!s.empty()) {
            int size = s.size();
            vector<int> tmp;
            while (size--) {
                if (level % 2 == 0) {
                    TreeNode* p = s.back();
                    s.pop_back();
                    tmp.push_back(p->val);
                    if (p->left) s.push_front(p->left);
                    if (p->right) s.push_front(p->right);
                } else {
                    TreeNode* p = s.front();
                    s.pop_front();
                    tmp.push_back(p->val);
                    if(p->right) s.push_back(p->right);
                    if(p->left) s.push_back(p->left);
                }
            }
            ret.push_back(tmp);
            level++;
        }
        return ret;
    }

};

也可以改变每层打印顺序，用vector的insert()方法