递归三序遍历二叉树

/**
 * struct TreeNode {
 *    int val;
 *    struct TreeNode *left;
 *    struct TreeNode *right;
 * };
 */

class Solution {
private:
    vector<vector<int>> res;
    vector<int> ans;
public:
    void PreOrder(TreeNode* root) {
        if (root == NULL) //递归终止条件
            return;
        else {
            ans.push_back(root->val);
            PreOrder(root->left);
            PreOrder(root->right);
        }
    }
    void InOrder(TreeNode* root) {
        if (root == NULL)
            return;
        else {
            InOrder(root->left);
            ans.push_back(root->val);
            //cout << root->val;//自测输出，判断是否出错
            InOrder(root->right);
        }
    }
    void PostOrder(TreeNode* root) {
        //ans.clear();//ans容器的清空，一定不能放在递归函数之内，否则会在每次递归时，将遍历后保存的数据清空，导致最后容器内空无一物。
        if (root == NULL)
            return ;
        else {
            PostOrder(root->left);
            PostOrder(root->right);
            ans.push_back(root->val);
            //cout << root->val;/自测输出
        }
    }
    vector<vector<int> > threeOrders(TreeNode* root) {
        // write code here
        if (root == NULL)
            return res;
        PreOrder(root);
        res.push_back(ans);
        ans.clear();//每次利用ans之前，需要将其清空，否则会将之前所得的循序保存
        InOrder(root);
        res.push_back(ans);
        ans.clear();
        PostOrder(root);
        res.push_back(ans);
        return res;
    }
};