二叉树的三种遍历方式

void preorder(struct TreeNode* root, int* res, int* resSize) {
    if (root == NULL) {
        return;
    }
    res[(*resSize)++] = root->val;
    preorder(root->left, res, resSize);
    preorder(root->right, res, resSize);
}

int* preorderTraversal(struct TreeNode* root, int* returnSize) {
    int* res = malloc(sizeof(int) * 2000);
    *returnSize = 0;
    preorder(root, res, returnSize);
    return res;
}

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */


/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
void Traversal(struct TreeNode* root,int *returnNum,int* returnSize )
{
    if(root==NULL)
    {
        return; 
    }
    //左
    Traversal(root->left,returnNum,returnSize);

    //根
    returnNum[*returnSize]   = root->val;       
    *returnSize = *returnSize + 1;

    //右
    Traversal(root->right,returnNum,returnSize);
}

int* inorderTraversal(struct TreeNode* root, int* returnSize)
{
    //树中节点数目在范围 [0, 100] 内
    int *returnNum = (int *)malloc(sizeof(int)*101);
    *returnSize = 0;
    if(root == NULL)
    {
        return NULL;
    }
    Traversal(root,returnNum,returnSize);
    return returnNum;
}

    void traversal(TreeNode* cur, vector<int>& vec) {
        if (cur == NULL) return;
        traversal(cur->left, vec);  // 左
        traversal(cur->right, vec); // 右
        vec.push_back(cur->val);    // 中
    }