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..<array.count {
var right = i
while right < array.count {
if valid(array,i,right){
count += 1
}
right += 1
}
}
return count
}
func valid(_ array:Array<Character>,_ 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
}
