func (this *Solution) removeDuplicates(str string, k int) string {
    stack := []rune{}
    for _, rune := range str {
        count := 0
        for len(stack) > count && rune == stack[len(stack)-count-1] {
            count++
        }
        if count == k-1 {
            stack = stack[:len(stack)-k+1]
            continue
        }
        stack = append(stack, rune)

    }
    return string(stack)
}