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BIT - Binary Indexed Tree (Fenwick Tree).cpp
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BIT - Binary Indexed Tree (Fenwick Tree).cpp
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struct BIT
{
int N;
vector<int> bit;
void init(int n)
{
N = n;
bit.assign(n + 1, 0);
}
void update(int idx, int val)
{
while(idx <= N)
{
bit[idx] += val;
idx += idx & -idx;
}
}
int pref(int idx)
{
int ans = 0;
while(idx > 0)
{
ans += bit[idx];
idx -= idx & -idx;
}
return ans;
}
int rsum(int l, int r)
{
return pref(r) - pref(l - 1);
}
//returns first index such that pref[idx] >= val
int search(int val)
{
int curSum = 0, pos = 0;
for(int i = log2(N) + 1; i >= 0; i--)
{
if(pos + (1 << i) < N && curSum + bit[pos + (1 << i)] < val)
{
curSum += bit[pos + (1 << i)];
pos += (1 << i);
}
}
return pos + 1;
}
void updateMax(int idx, int val)
{
while(idx <= N)
{
bit[idx] = max(bit[idx], val);
idx += idx & -idx;
}
}
int prefMax(int idx)
{
int ans = -2e9;
while(idx > 0)
{
ans = max(ans, bit[idx]);
idx -= idx & -idx;
}
return ans;
}
};
//Problem 1: https://codeforces.com/contest/1073/problem/D
//Solution 1: https://codeforces.com/contest/1073/submission/44863255
//Problem 2: https://www.hackerrank.com/contests/university-codesprint-4/challenges/unique-art/problem
//Solution 2: http://p.ip.fi/T9YM
//Problem 3: https://csacademy.com/contest/round-47/task/cut-the-trees/statement/