Project Euler 014 Longest Collatz sequence

#include <bits/stdc++.h>

#define ll long long

#define pii std::pair<int,int>
#define mp std::make_pair
#define fi first
#define se second

#define SZ(x) (int)(x).size()
#define pb push_back

template<class T>inline void chkmax(T &x, const T &y) {if(x < y) x = y;}
template<class T>inline void chkmin(T &x, const T &y) {if(x > y) x = y;}

template<class T>
inline void read(T &x) {
char c;int f = 1;x = 0;
while(((c=getchar()) < '0' || c > '9') && c != '-');
if(c == '-') f = -1;else x = c-'0';
while((c=getchar()) >= '0' && c <= '9') x = x*10+c-'0';
x *= f;
}
static int outn;
static char out[(int)2e7];
template<class T>
inline void write(T x) {
if(x < 0) out[outn++] = '-', x = -x;
if(x) {
static int tmpn;
static char tmp[20];
tmpn = 0;
while(x) tmp[tmpn++] = x%10+'0', x /= 10;
while(tmpn) out[outn++] = tmp[--tmpn];
}
else out[outn++] = '0';
}

std::unordered_map<ll, int> f;

int go(ll x) {
if(__builtin_popcount(x) == 1) return __builtin_ctz(x);
if(f.count(x)) return f[x];
if(x & 1) return f[x] = go(3 * x + 1) + 1;
else return f[x] = go(x >> 1) + 1;
}

int main() {
int max = 0, maxi = -1;
for(int i = 1; i <= 1000000; ++i) {
int cur = go(i);
if(max < cur) max = cur, maxi = i;
}
printf("%d\n", maxi);
return 0;
}