剑指 Offer II 020. 回文子字符串的个数

有点滑动窗口的意思。。
valid判断是否是回文字符串

func countSubstrings(_ s: String) -> Int {
        let array = Array(s)
        var count = 0
        
        for i in 0..,_ left:Int ,_ right:Int) -> Bool {
        var  start = left
        var  end = right
        while start < end {
            
            if  array[start] == array[end] {
                start += 1
                end -= 1
            }else {
                return false
            }
            
        }
        
        return true
        
        
    }