LeetCode OJ 145. Binary Tree Postorder Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
/**
 * Return an array of size *returnSize.
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* postorderTraversal(struct TreeNode* root, int* returnSize) {
    int stack[1000], top = -1, i = 0;
    int *return_array = (int*)malloc(sizeof(int) * 1000);
    struct TreeNode *visit = NULL;
    
    while(-1 != top||NULL != root){
        while(NULL != root){
            stack[++top] = root;
            root = root->left;
        }
        root = stack[top];
        if(NULL == root->right||visit == root->right){
            return_array[i++] = root->val;
            top--;
            visit = root;
            root = NULL;
        }
        else{
            root = root->right;
        }
    }
    *returnSize = i;
    return return_array;
}