力扣144. 二叉树的前序遍历145. 二叉树的后序遍历94. 二叉树的中序遍历（递归版）

前序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void getPreorderTraversal(TreeNode* root, vector<int>& result) {
        if(root == nullptr) return;

        result.push_back(root->val);
        getPreorderTraversal(root->left, result);
        getPreorderTraversal(root->right, result);
    }

    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        getPreorderTraversal(root, result);

        return result;
    }
};

后序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void getPostorderTraversal(TreeNode* root, vector<int>& result) {
        if(root == nullptr) return;

        getPostorderTraversal(root->left, result);
        getPostorderTraversal(root->right, result);
        result.push_back(root->val);
    }

    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> result;
        getPostorderTraversal(root, result);

        return result;
    }
};

中序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void getInorderTraversal(TreeNode* root, vector<int>& result) {
        if(root == nullptr) return;

        getInorderTraversal(root->left, result);
        result.push_back(root->val);
        getInorderTraversal(root->right, result);
    }

    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> result;
        getInorderTraversal(root, result);

        return result;
    }
};