题解 | #实现二叉树先序，中序和后序遍历#

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

class Solution {
public:
    
    void preOrder(TreeNode* root,vector<int> &ans){
        if(root == nullptr) return;
        ans.insert(ans.end(), root->val);
        preOrder(root->left, ans);
        preOrder(root->right, ans);
    }
    
    void postOrder(TreeNode* root,vector<int> &ans){
        if(root == nullptr) return;
        postOrder(root->left, ans);
        postOrder(root->right, ans);
        ans.insert(ans.end(), root->val);
    }
    
    void midOrder(TreeNode* root,vector<int> &ans){
        if(root == nullptr) return;
        midOrder(root->left, ans);
        ans.insert(ans.end(), root->val);
        midOrder(root->right, ans);
    }
    /**
     * 
     * @param root TreeNode类 the root of binary tree
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > threeOrders(TreeNode* root) {
        vector<vector<int>> ans(3);
        preOrder(root, ans[0]);
        midOrder(root, ans[1]);
        postOrder(root, ans[2]);
        return ans;
        
    }
    
    
};