题解 | #实现二叉树先序，中序和后序遍历#

用最基本的前序，中序，后续，然后传入数组引用，注意递归返回值，注意函数调用

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 the root of binary tree
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > threeOrders(TreeNode* root) {
        // write code here
        vector<vector<int> > results;

         vector<int> pre;
         pre_order(root,pre);
         results.push_back(pre);
         vector<int> in;
         in_order(root,in);
         results.push_back(in);
         vector<int> pos; 
         pos_order(root,pos);
        results.push_back(pos);

        return results;

    }
    void pre_order(TreeNode* root, vector<int> &a){
        if(!root){
            return;
        }

        a.push_back(root->val);
        pre_order(root->left,a);
        pre_order(root->right,a);
    }

    void in_order(TreeNode* root, vector<int> &a){

         if(!root){
            return;
        }


        in_order(root->left,a);
        a.push_back(root->val);
        in_order(root->right,a);
    }

     void pos_order(TreeNode* root, vector<int> &a){

         if(!root){
            return;
        }

        pos_order(root->left,a);
        pos_order(root->right,a);
        a.push_back(root->val);
    }



};