分别调用三个函数，分别先序遍历、中序遍历、后序遍历。前序遍历先向结果数组 ret 中插入根节点值，然后递归调用左、右子树的前序遍历函数，中序遍历和后序遍历类似。代码如下：
/**
* struct TreeNode {
* int val;
* struct TreeNode *left;
* struct TreeNode *right;
* };
*/
class Solution {
vector<vector<int> > ret;
public:
/**
*
* @param root TreeNode类 the root of binary tree
* @return int整型vector<vector<>>
*/
void preorder(TreeNode *root) {
if(root == NULL) return;
ret[0].push_back(root->val);
preorder(root->left);
preorder(root->right);
}
void inorder(TreeNode *root) {
if(root == NULL) return;
inorder(root->left);
ret[1].push_back(root->val);
inorder(root->right);
}
void afterorder(TreeNode *root) {
if(root == NULL) return;
afterorder(root->left);
afterorder(root->right);
ret[2].push_back(root->val);
}
vector<vector<int> > threeOrders(TreeNode* root) {
ret.resize(3);
preorder(root);
inorder(root);
afterorder(root);
return ret;
}
};
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