1：int sum = ((1+n)*n)/2 ;
2: int sum = (n + 1) * n / 2;
3： int sum = 0;
for (int i =1; i <= n; i++) {
sum += i;
}
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哈喽王小熊
1：int sum = ((1+n)*n)/2 ;
2: int sum = (n + 1) * n / 2;
3： int sum = 0;
for (int i =1; i <= n; i++) {
sum += i;
}