315. Count of Smaller Numbers After Self

Description

Given an integer array nums, return an integer array counts where counts[i] is the number of smaller elements to the right of nums[i].

Example 1:

Input: nums = [5,2,6,1]
Output: [2,1,1,0]
Explanation:
To the right of 5 there are 2 smaller elements (2 and 1).
To the right of 2 there is only 1 smaller element (1).
To the right of 6 there is 1 smaller element (1).
To the right of 1 there is 0 smaller element.

Example 2:

Input: nums = [-1]
Output: [0]

Example 3:

Input: nums = [-1,-1]
Output: [0,0]

Constraints:

1 <= nums.length <= 10⁵
-10⁴ <= nums[i] <= 10⁴

Solutions

Solution 1

Python3JavaC++Go

class BinaryIndexedTree:
    def __init__(self, n):
        self.n = n
        self.c = [0] * (n + 1)

    @staticmethod
    def lowbit(x):
        return x & -x

    def update(self, x, delta):
        while x <= self.n:
            self.c[x] += delta
            x += BinaryIndexedTree.lowbit(x)

    def query(self, x):
        s = 0
        while x > 0:
            s += self.c[x]
            x -= BinaryIndexedTree.lowbit(x)
        return s


class Solution:
    def countSmaller(self, nums: List[int]) -> List[int]:
        alls = sorted(set(nums))
        m = {v: i for i, v in enumerate(alls, 1)}
        tree = BinaryIndexedTree(len(m))
        ans = []
        for v in nums[::-1]:
            x = m[v]
            tree.update(x, 1)
            ans.append(tree.query(x - 1))
        return ans[::-1]

class Solution {
    public List<Integer> countSmaller(int[] nums) {
        Set<Integer> s = new HashSet<>();
        for (int v : nums) {
            s.add(v);
        }
        List<Integer> alls = new ArrayList<>(s);
        alls.sort(Comparator.comparingInt(a -> a));
        int n = alls.size();
        Map<Integer, Integer> m = new HashMap<>(n);
        for (int i = 0; i < n; ++i) {
            m.put(alls.get(i), i + 1);
        }
        BinaryIndexedTree tree = new BinaryIndexedTree(n);
        LinkedList<Integer> ans = new LinkedList<>();
        for (int i = nums.length - 1; i >= 0; --i) {
            int x = m.get(nums[i]);
            tree.update(x, 1);
            ans.addFirst(tree.query(x - 1));
        }
        return ans;
    }
}

class BinaryIndexedTree {
    private int n;
    private int[] c;

    public BinaryIndexedTree(int n) {
        this.n = n;
        c = new int[n + 1];
    }

    public void update(int x, int delta) {
        while (x <= n) {
            c[x] += delta;
            x += lowbit(x);
        }
    }

    public int query(int x) {
        int s = 0;
        while (x > 0) {
            s += c[x];
            x -= lowbit(x);
        }
        return s;
    }

    public static int lowbit(int x) {
        return x & -x;
    }
}

class BinaryIndexedTree {
public:
    int n;
    vector<int> c;

    BinaryIndexedTree(int _n)
        : n(_n)
        , c(_n + 1) {}

    void update(int x, int delta) {
        while (x <= n) {
            c[x] += delta;
            x += lowbit(x);
        }
    }

    int query(int x) {
        int s = 0;
        while (x > 0) {
            s += c[x];
            x -= lowbit(x);
        }
        return s;
    }

    int lowbit(int x) {
        return x & -x;
    }
};

class Solution {
public:
    vector<int> countSmaller(vector<int>& nums) {
        unordered_set<int> s(nums.begin(), nums.end());
        vector<int> alls(s.begin(), s.end());
        sort(alls.begin(), alls.end());
        unordered_map<int, int> m;
        int n = alls.size();
        for (int i = 0; i < n; ++i) m[alls[i]] = i + 1;
        BinaryIndexedTree* tree = new BinaryIndexedTree(n);
        vector<int> ans(nums.size());
        for (int i = nums.size() - 1; i >= 0; --i) {
            int x = m[nums[i]];
            tree->update(x, 1);
            ans[i] = tree->query(x - 1);
        }
        return ans;
    }
};

type BinaryIndexedTree struct {
    n int
    c []int
}

func newBinaryIndexedTree(n int) *BinaryIndexedTree {
    c := make([]int, n+1)
    return &BinaryIndexedTree{n, c}
}

func (this *BinaryIndexedTree) lowbit(x int) int {
    return x & -x
}

func (this *BinaryIndexedTree) update(x, delta int) {
    for x <= this.n {
        this.c[x] += delta
        x += this.lowbit(x)
    }
}

func (this *BinaryIndexedTree) query(x int) int {
    s := 0
    for x > 0 {
        s += this.c[x]
        x -= this.lowbit(x)
    }
    return s
}

func countSmaller(nums []int) []int {
    s := make(map[int]bool)
    for _, v := range nums {
        s[v] = true
    }
    var alls []int
    for v := range s {
        alls = append(alls, v)
    }
    sort.Ints(alls)
    m := make(map[int]int)
    for i, v := range alls {
        m[v] = i + 1
    }
    ans := make([]int, len(nums))
    tree := newBinaryIndexedTree(len(alls))
    for i := len(nums) - 1; i >= 0; i-- {
        x := m[nums[i]]
        tree.update(x, 1)
        ans[i] = tree.query(x - 1)
    }
    return ans
}

Solution 2