Compute alternating sum

#[test]
fn t1() {
    let start = std::time::Instant::now();
    let val = Solution::alternating_sum(vec![1, 3, 5, 7]);
    let expected = -4;
    log_result!(val == expected, "t1", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t2() {
    let start = std::time::Instant::now();
    let val = Solution::alternating_sum(vec![100]);
    let expected = 100;
    log_result!(val == expected, "t2", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t3() {
    let start = std::time::Instant::now();
    let val = Solution::alternating_sum(vec![1, 1]);
    let expected = 0;
    log_result!(val == expected, "t3", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t4() {
    let start = std::time::Instant::now();
    let val = Solution::alternating_sum(vec![5, 2, 3]);
    let expected = 6;
    log_result!(val == expected, "t4", start, &expected, &val);
    assert_eq!(val, expected);
}

#[test]
fn t5() {
    let start = std::time::Instant::now();
    let val = Solution::alternating_sum(vec![10, 5, 10, 5, 10]);
    let expected = 20;
    log_result!(val == expected, "t5", start, &expected, &val);
    assert_eq!(val, expected);
}