【LeetCode 327】区间和的个数

输入：
nums = [-2,5,-1], lower = -2, upper = 2,
输出：
3
解释：
3个区间分别是: [0,0], [2,2], [0,2]，它们表示的和分别为: -2, -1, 2。

typedef long long ll;

class BIT {
private:
    static const int MAXN = 100010;
    int bit[MAXN];

public:
    BIT() {
        memset(bit, 0, sizeof bit);
    }

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

    void add(int i, int x) {
        while (i < MAXN) {
            bit[i] += x;
            i += lowbit(i);
        }
    }

    void sub(int i, int x) {
        while (i < MAXN) {
            bit[i] -= x;
            i += lowbit(i);
        }
    }

    int sum(int i) {
        int s = 0;
        while (i > 0) {
            s += bit[i];
            i -= lowbit(i);
        }
        return s;
    }
};

class ID {
private:
    unordered_map<ll, int> mp;
    set<ll> st;
    int idx;

public:
    ID() {
        mp.clear();
        st.clear();
        idx = 1;
    }

    void addNum(ll x) {
        st.insert(x);
    }

    void proj() {
        for (ll x: st) {
            mp[x] = idx++;
        }
    }

    int getID(ll x) {
        return mp[x];
    }
};

class Solution {
public:
    int countRangeSum(vector<int>& nums, int lower, int upper) {
        int n = nums.size();
        ll sum = 0, res = 0;
        ID id = ID();
        BIT bit = BIT();
        
        id.addNum(0);
        for (int i = 0; i < n; ++i) {
            sum += nums[i];
            id.addNum(sum);
            id.addNum(sum-lower);
            id.addNum(sum-upper);
        }
        id.proj();
        
        bit.add(id.getID(0), 1);
        sum = 0;
        for (int i = 0; i < n; ++i) {
            sum += nums[i];
            int lb = id.getID(sum-upper), rb = id.getID(sum-lower);
            res += bit.sum(rb) - bit.sum(lb-1);
            bit.add(id.getID(sum), 1);
        }
        return res;
    }
};