func isValid(s string) bool {
    if len(s) == 0 || len(s) % 2 == 1 {
        return false
    }

    pairs := map[rune]rune {
        '(':')',
        '{':'}',
        '[':']',
    }
    stack:=[]rune{}

    for _,r:= range s {
        if _, ok:=pairs[r]; ok {
            stack = append(stack,r)
        }else if len(stack) == 0 || pairs[stack[len(stack)-1]] != r {
            return false
        }else {
            stack = stack[:len(stack)-1]
        }
    }
    return len(stack) == 0
}