https://www.hackerrank.com/challenges/bigger-is-greater

BiggerIsGreater.go

package main

import (
	"bufio"
	"fmt"
	"os"
)

func Reverse(a []byte) {
	n := len(a)
	for j := 0; j < n/2; j++ {
		tmp := a[j]
		a[j] = a[n-j-1]
		a[n-j-1] = tmp
	}
}

func NextPerm(sPtr *string) string {
	b := []byte(*sPtr)

	n := len(b)
	for i := n - 2; i >= 0; i-- {
		if b[i] < b[i+1] {
			// Find the last element that is greater than b[i]
			nextMin := n - 1
			for ; b[nextMin] <= b[i]; nextMin-- {
			}

			// swap i, nextMin
			tmp := b[i]
			b[i] = b[nextMin]
			b[nextMin] = tmp

			// reverse i+1 .. n
			Reverse(b[i+1 : n])

			return string(b)
		}
	}

	return "no answer"
}

func main() {
	in := bufio.NewReader(os.Stdin)

	var t int
	fmt.Fscanf(in, "%d\n", &t)

	for ; t > 0; t-- {
		var s string
		fmt.Fscanf(in, "%s\n", &s)

		fmt.Println(NextPerm(&s))
	}
}