给定一棵二叉树，使用递归和非递归的方法实现二叉树的中序遍历结果。C语言

递归实现二叉树中序遍历：

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

void inorderTraversalHelper(struct TreeNode* root, int* result, int* index) {
    if (!root) {
        return;
    }
    inorderTraversalHelper(root->left, result, index);
    result[(*index)++] = root->val;
    inorderTraversalHelper(root->right, result, index);
}

int* inorderTraversal(struct TreeNode* root, int* returnSize){
    int* result = (int*)malloc(sizeof(int) * 100);
    memset(result, 0, sizeof(int) * 100);
    *returnSize = 0;
    inorderTraversalHelper(root, result, returnSize);
    return result;
}

非递归实现二叉树中序遍历：

#define MAX_SIZE 100

int* inorderTraversal(struct TreeNode* root, int* returnSize){
    int* result = (int*)malloc(sizeof(int) * 100);
    memset(result, 0, sizeof(int) * 100);
    *returnSize = 0;
    struct TreeNode* stack[MAX_SIZE];
    int top = -1;
    while (root || top != -1) {
        while (root) {
            stack[++top] = root;
            root = root->left;
        }
        if (top != -1) {
            root = stack[top--];
            result[(*returnSize)++] = root->val;
            root = root->right;
        }
    }
    return result;
}