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;
 *  TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 the root of binary tree
     * @return int整型vector<vector<>>
     */
    vector<int> a;
    vector<int> b;
    vector<int> c;

    // 前序遍历，先访问根节点，再递归遍历左子树和右子树
    void preorder(TreeNode* root) {
        if (root == nullptr) {
            return;
        }
        a.push_back(root->val);
        preorder(root->left);
        preorder(root->right);
    }

    // 中序遍历，先递归遍历左子树，再访问根节点，最后递归遍历右子树
    void preorder1(TreeNode* root) {
        if (root == nullptr) {
            return;
        }
        preorder1(root->left);
        b.push_back(root->val);
        preorder1(root->right);
    }

    // 后序遍历，先递归遍历左子树和右子树，再访问根节点
    void preorder2(TreeNode* root) {
        if (root == nullptr) {
            return;
        }
        preorder2(root->left);
        preorder2(root->right);
        c.push_back(root->val);
    }

    vector<vector<int>> threeOrders(TreeNode* root) {
        // write code here
        vector<vector<int>> ss;
        preorder(root);
        preorder1(root);
        preorder2(root);
        ss.push_back(a);
        ss.push_back(b);
        ss.push_back(c);
        return ss;
    }
};