1 vector<vector<int>> levelOrder(TreeNode* root){ 2 vector<vector<int>> res; 3 if(root==NULL) return res; 4 bool flag = true;//true--输出奇层，放入偶层（左右）－（前出后进）false--输出偶层，放入奇层（右左）－（后出前放） 5 6 deque<TreeNode*> q; 7 q.push_back(root);//第一层已经入队 8 9 while(!q.empty()){ 10 int n = q.size(); 11 vector<int> out; 12 TreeNode* node; 13 14 while(n>0){ 15 if(flag){//前取后放：第一层为奇数 16 node = q.front(); 17 q.pop_front(); 18 if(node->left) 19 q.push_back(node->left);//放入下一层－－偶数层：后放－－左右 20 if(node->right) 21 q.push_back(node->right) 22 } 23 else{//后取前放：该层为偶层－－后取 24 node = q,back(); 25 q.pop_back(); 26 if(node->right) 27 q.push_front(node->right);//放入下一层－－奇层 28 if(node->left) 29 q.push_front(node->left; 30 } 31 out.push_back(node->val); 32 n--; 33 } 34 flag = !flag; 35 res.push_back(out); 36 } 37 return res; 38 }