void extgcd(int a, int b, int &x1, int &y1){ if(b == 0){ x1 = 1; y1 = 0; }else{ int x2,y2; extgcd(b, a%b, x2, y2); x1 = y2; y1 = x2 - a / b * y2; } }
(x+y)n=C0nx0yn+C1nx1yn−1+C2nx2yn−2+...+Cnnxny0
Ckn=n!k!(n−k)!
or
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