2022-05-25：最大子段和是 一个经典问题，即对于一个数组找出其和最大的子数组。 现在允许你在求解该问题之前翻转这个数組的连续一段， 如翻转(1,2,3,

use rand::Rng;
fn main() {
    let len: i32 = 5;
    let value: i32 = 20;
    let test_time: i32 = 20000;
    println!("测试开始");
    for _i in 0..test_time {
        let n: i32 = rand::thread_rng().gen_range(0, len) + 1;
        let mut arr = random_array(n, value);
        let ans1 = max_sum_reverse1(&mut arr);
        let ans2 = max_sum_reverse2(&mut arr);
        if ans1 != ans2 {
            println!("出错了!");
            for num in &arr {
                print!("{} ", num);
            }
            println!("");
            println!("ans1 = {}", ans1);
            println!("ans2 = {}", ans2);
            break;
        }
    }
    println!("测试结束");
}

fn max_sum_reverse1(arr: &mut Vec<i32>) -> i32 {
    let mut ans: i32 = i32::min_value();
    for l in 0..arr.len() as i32 {
        for r in l..arr.len() as i32 {
            reverse(arr, l, r);
            ans = get_max(ans, max_sum(arr));
            reverse(arr, l, r);
        }
    }
    return ans;
}

fn reverse(arr: &mut Vec<i32>, mut l: i32, mut r: i32) {
    while l < r {
        let tmp = arr[l as usize];
        arr[l as usize] = arr[r as usize];
        l += 1;
        arr[r as usize] = tmp;
        r -= 1;
    }
}

fn max_sum(arr: &mut Vec<i32>) -> i32 {
    let mut pre = arr[0];
    let mut max = arr[0];
    for i in 1..arr.len() as i32 {
        pre = get_max(arr[i as usize], arr[i as usize] + pre);
        max = get_max(max, pre);
    }
    return max;
}

fn get_max<T: Clone + Copy + std::cmp::PartialOrd>(a: T, b: T) -> T {
    if a > b {
        a
    } else {
        b
    }
}

fn max_sum_reverse2(arr: &mut Vec<i32>) -> i32 {
    let n = arr.len() as i32;
    let mut prefix: Vec<i32> = vec![];
    for _i in 0..n {
        prefix.push(0)
    }
    prefix[(n - 1) as usize] = arr[(n - 1) as usize];
    let mut i: i32 = n - 2;
    while i >= 0 {
        prefix[i as usize] = arr[i as usize] + get_max(0, prefix[(i + 1) as usize]);
        i -= 1;
    }
    let mut ans = prefix[0];
    let mut suffix = arr[0];
    let mut max_suffix = suffix;
    for i in 1..n {
        ans = get_max(ans, max_suffix + prefix[i as usize]);
        suffix = arr[i as usize] + get_max(0, suffix);
        max_suffix = get_max(max_suffix, suffix);
    }
    ans = get_max(ans, max_suffix);
    return ans;
}

// 为了测试
fn random_array(n: i32, v: i32) -> Vec<i32> {
    let mut arr: Vec<i32> = vec![];
    for _i in 0..n {
        arr.push(rand::thread_rng().gen_range(0, v) - rand::thread_rng().gen_range(0, v));
    }
    return arr;
}