【leetcode】95. 不同的二叉搜索树 II

struct TreeNode** buildTree(int start, int end, int* returnSize) {
    if (start > end) {
        (*returnSize) = 1;
        struct TreeNode** ret = malloc(sizeof(struct TreeNode*));
        ret[0] = NULL;
        return ret;
    }
    *returnSize = 0;
    struct TreeNode** allTrees = malloc(0);
    // 枚举可行根节点
    for (int i = start; i <= end; i++) {
        // 获得所有可行的左子树集合
        int leftTreesSize;
        struct TreeNode** leftTrees = buildTree(start, i - 1, &leftTreesSize);

        // 获得所有可行的右子树集合
        int rightTreesSize;
        struct TreeNode** rightTrees = buildTree(i + 1, end, &rightTreesSize);

        // 从左子树集合中选出一棵左子树，从右子树集合中选出一棵右子树，拼接到根节点上
        for (int left = 0; left < leftTreesSize; left++) {
            for (int right = 0; right < rightTreesSize; right++) {
                struct TreeNode* currTree = malloc(sizeof(struct TreeNode));
                currTree->val = i;
                currTree->left = leftTrees[left];
                currTree->right = rightTrees[right];

                (*returnSize)++;
                allTrees = realloc(allTrees, sizeof(struct TreeNode*) * (*returnSize));
                allTrees[(*returnSize) - 1] = currTree;
            }
        }
        free(rightTrees);
        free(leftTrees);
    }
    return allTrees;
}

struct TreeNode** generateTrees(int n, int* returnSize) {
    if (!n) {
        (*returnSize) = 0;
        return NULL;
    }
    return buildTree(1, n, returnSize);
}