CF 2149D

package main

import (
    "fmt"
)

const N = 200005

var (
    sum_l   [N]int
    sum_r   [N]int
    sum_2_l [N]int
    sum_2_r [N]int
)

func solve() {
    var n int
    var s string
    ans := 99999999999
    fmt.Scan(&n)
    fmt.Scan(&s)
    for i := 0; i < n; i++ {
        sum_l[i], sum_r[i], sum_2_l[i], sum_2_r[i] = 0, 0, 0, 0
    }
    if len(s) == 1 {
        fmt.Println(0)
        return
    }
    count := 0
    if s[0] == 'a' {
        count = 1
    }
    for i := 1; i < n; i++ {
        if s[i] == 'a' {
            sum_l[i] = sum_l[i-1] + i - count
            count++
        } else {
            sum_l[i] = sum_l[i-1]
        }
    }
    count = 0
    if s[n-1] == 'a' {
        count = 1
    }
    for i := n - 2; i >= 0; i-- {
        if s[i] == 'a' {
            sum_r[i] = sum_r[i+1] + n - (i + 1) - count
            count++
        } else {
            sum_r[i] = sum_r[i+1]
        }
    }
    for i := 0; i < n-1; i++ {
        now := sum_l[i] + sum_r[i+1]
        if now < ans {
            ans = now
        }
    }

    count = 0
    if s[0] == 'a' {
        count++
    }
    for i := 1; i < n; i++ {
        if s[i] == 'a' {
            sum_2_l[i] = sum_2_l[i-1]
            count++
        } else {
            sum_2_l[i] = sum_2_l[i-1] + count
        }
    }

    count = 0
    if s[n-1] == 'a' {
        count++
    }
    for i := n - 2; i >= 0; i-- {
        if s[i] == 'a' {
            sum_2_r[i] = sum_2_r[i+1]
            count++
        } else {
            sum_2_r[i] = sum_2_r[i+1] + count
        }
    }

    for i := 0; i <= n-1; i++ {
        now := sum_2_l[i] + sum_2_r[i]
        if now < ans {
            ans = now
        }
    }
    fmt.Println(ans)
}

func main() {
    var t int
    fmt.Scan(&t)
    for i := 0; i < t; i++ {
        solve()
    }
}

package main

import (
    "fmt"
)

func AbsInt(x int) int {
    if x < 0 {
        return -x
    }
    return x
}

func calc(vector []int) int {
    if len(vector) == 0 || len(vector) == 1 {
        return 0
    }
    ans := 0
    pos := len(vector) / 2
    for i := 0; i < len(vector); i++ {
        ans += AbsInt(vector[pos]-vector[i]) - AbsInt(pos-i)
    }
    return ans
}

func solve() {
    var n int
    var str string
    var pos_a []int
    var pos_b []int
    fmt.Scan(&n, &str)
    for i := 0; i < n; i++ {
        if str[i] == 'a' {
            pos_a = append(pos_a, i)
        } else {
            pos_b = append(pos_b, i)
        }
    }
    l_ans := calc(pos_a)
    r_ans := calc(pos_b)
    var ans int
    if l_ans > r_ans {
        ans = r_ans
    } else {
        ans = l_ans
    }
    fmt.Println(ans)
}

func main() {
    var t int
    fmt.Scan(&t)
    for i := 0; i < t; i++ {
        solve()
    }
}