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void cycleDetection(ll x0, function<ll(ll)> f) {
ll a = x0, b = f(x0), length = 1;
for (ll power = 1; a != b; b = f(b), length++) {
if (power == length) {
power *= 2;
length = 0;
a = b;
}}
ll start = 0;
a = x0; b = x0;
for (ll i = 0; i < length; i++) b = f(b);
while (a != b) {
a = f(a);
b = f(b);
start++;
}}
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