剑指Offer-07斐波那契数列

public static int fib(int n) {
    if (n < 2){
        return n;
    }
    int a = 0;
    int b = 1;
    int sum = a + b;
    //a b sum 每次循环用前两个变量保留b和sum
    for(int i = 2; i < n ; i++){
        a = b;
        b = sum;
        sum = (a + b) % 1000000007;
    }
    return sum;
}