```go
package main

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

func main() {
	reader := bufio.NewReader(os.Stdin)
	writer := bufio.NewWriter(os.Stdout)
	defer writer.Flush()

	var t int
	fmt.Fscan(reader, &t)

	for ; t > 0; t-- {
		var n int
		fmt.Fscan(reader, &n)
		var b string
		fmt.Fscan(reader, &b)

		// Find distinct characters and sort them
		seen := make(map[byte]bool)
		for i := 0; i < len(b); i++ {
			seen[b[i]] = true
		}
		r := []byte{}
		for c := range seen {
			r = append(r, c)
		}
		sort.Slice(r, func(i, j int) bool { return r[i] < r[j] })

		// Build mapping: symmetric replacement
		mapping := make(map[byte]byte)
		for i := 0; i < len(r); i++ {
			mapping[r[i]] = r[len(r)-1-i]
		}

		result := make([]byte, n)
		for i := 0; i < n; i++ {
			result[i] = mapping[b[i]]
		}

		fmt.Fprintf(writer, "%s\n", string(result))
	}
}
```