题解 | #按之字形顺序打印二叉树#

层次遍历，借助辅助栈

奇数层的直接加入vector

偶数层加入stack，然后再弹栈到vector中

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>> res;
        bool flag=true;
        if(pRoot==NULL) return res;
        queue<TreeNode*> q;
        q.push(pRoot);
        while(!q.empty()){
            int size=q.size();
            vector<int> ve;
            stack<int> st;
            while(size--){
                TreeNode* node=q.front();
                q.pop();
                if(flag) ve.push_back(node->val);
                else st.push(node->val);
                if(node->left) q.push(node->left);
                if(node->right) q.push(node->right);
            }
            if(!flag){
                while(!st.empty()){
                    ve.push_back(st.top());
                    st.pop();
                }
            }
            if(ve.size()>0) res.push_back(ve);
            flag=!flag;
        }
        return res;
    }
    
};