Leetcode 315 Solution

This article provides solution to leetcode question 315 (count-of-smaller-numbers-after-self)
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https://leetcode.com/problems/count-of-smaller-numbers-after-self

Solution

struct Node
{
    int m_val;
    int smaller;
    int count = 1;
    Node* left;
    Node* right;

    Node(int val)
    {
        m_val = val;
        smaller = 0;
        count = 1;
        left = NULL;
        right = NULL;
    }
};

class BST
{
    Node* m_root;

    Node* insert(Node* node, int val)
    {
        if (node == NULL)
            return new Node(val);
        else if (val == node->m_val)
        {
            node->count++;
            return node;
        }
        else if (val < node->m_val)
        {
            node->smaller++;
            node->left = insert(node->left, val);
            return node;
        }
        else if (val > node->m_val)
        {
            node->right = insert(node->right, val);
            return node;
        }

        return NULL;
    }

    int findsmaller(Node* node, int val)
    {
        if (node == NULL)
            return 0;
        else if (node->m_val == val)
            return node->smaller;
        else if (node->m_val > val)
            return findsmaller(node->left, val);
        else if (node->m_val < val)
            return node->smaller + node->count + findsmaller(node->right, val);

        return 0;
    }

public:
    BST()
    {
        m_root = NULL;
    }

    void insert(int val)
    {
        m_root = insert(m_root, val);
    }

    int findsmaller(int val)
    {
        return findsmaller(m_root, val);
    }
};

class Solution {
public:
    vector<int> countSmaller(vector<int>& nums) {
        BST bst;
        vector<int> res(nums.size());

        for (int i = nums.size() - 1; i >= 0; i--)
        {
            res[i] = bst.findsmaller(nums[i]);
            bst.insert(nums[i]);
        }

        return res;
    }
};