int a = 0, b = 0, c = 0;分别为 SA SB SC 的下标
while(a < length_of_SA && b < length_of_SB)
{
if( SA[a] < SB[b])
SC[c++] = SA[a++];
else
SC[c++] = SB[b++];
}
if(a < length_of_SA)
for(; a < length_of_SA;)
SC[c++] = SA[a++];
else if(b < length_of_SB)
for(; b < length_of_SB;)
SC[c++] = SB[b++];