func removeDuplicates(s string) string {
var stack []rune
for _, c := range s {
if len(stack) > 0 && stack[len(stack)-1] == c {
stack = stack[:len(stack)-1]
} else {
stack = append(stack, c)
}
}
var ans string
for ;len(stack)>0; {
c, stac := stack[len(stack)-1], stack[:len(stack)-1]
stack = stac
ans = string(c) + ans
}
return ans
}