Greatest common divisor(gcd)

欧几里得算法求最大公约数

• If A = 0 then GCD(A,B)=B, since the GCD(0,B)=B, and we can stop.  
• If B = 0 then GCD(A,B)=A, since the GCD(A,0)=A, and we can stop.  
• Write A in quotient remainder form (A = B⋅Q + R)
• Find GCD(B,R) using the Euclidean Algorithm since GCD(A,B) = GCD(B,R)

这里Q是正整数.

Example:

Find the GCD of 270 and 192

• A=270, B=192
• A ≠0
• B ≠0
• Use long division to find that 270/192 = 1 with a remainder of 78. We can write this as: 270 = 192 * 1 +78
• Find GCD(192,78), since GCD(270,192)=GCD(192,78)

    A=192, B=78

• A ≠0
• B ≠0
• Use long division to find that 192/78 = 2 with a remainder of 36. We can write this as:
• 192 = 78 * 2 + 36
• Find GCD(78,36), since GCD(192,78)=GCD(78,36)

    A=78, B=36

• A ≠0
• B ≠0
• Use long division to find that 78/36 = 2 with a remainder of 6. We can write this as:
• 78 = 36 * 2 + 6
• Find GCD(36,6), since GCD(78,36)=GCD(36,6)

    A=36, B=6

• A ≠0
• B ≠0
• Use long division to find that 36/6 = 6 with a remainder of 0. We can write this as:
• 36 = 6 * 6 + 0
• Find GCD(6,0), since GCD(36,6)=GCD(6,0)

A=6, B=0

• A ≠0
• B =0, GCD(6,0)=6

So we have shown:

GCD(270,192) = GCD(192,78) = GCD(78,36) = GCD(36,6) = GCD(6,0) = 6

GCD(270,192) = 6

应用：

int gcd(int a, int b) {
    while(b){
       int r = a % b;
       a = b; 
       b = r;
   }
   return a;
}