题目链接：1339. 分裂二叉树的最大乘积

DFS两次即可

#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define MOD ((int)pow(10, 9) + 7)

static void post_order_traversal(struct TreeNode* node)
{
    if (!node) {
        return;
    }
    post_order_traversal(node->left);
    post_order_traversal(node->right);
    if (node->left) {
        node->val += node->left->val;
    }
    if (node->right) {
        node->val += node->right->val;
    }
}

static void dfs(struct TreeNode* node, const long long sum, long long *ans)
{
    if (!node) {
        return;
    }
    long long a = node->val;
    long long b = sum - node->val;
    *ans = MAX(*ans, (a * b));
    dfs(node->left, sum, ans);
    dfs(node->right, sum, ans);
}

int maxProduct(struct TreeNode* root){
    post_order_traversal(root);
    long long sum = root->val;
    long long ans = 0;
    dfs(root, sum, &ans);
    return (ans % MOD);
}