2．　　链栈的应用：编写函数，判断给定的字符串是否回文。如字符串“abcba”、“abccba”均为中心对称，字符串“abcdba”不中心对称，我们将中心对称的字符串称为回文。-2. Chain stack applications: Write a function to determine whether a given string palindrome. If the string " abcba" , " abccba"　are the center of symmetry, the string " abcdba"　no center of symmetry, we will centrosymmetric string called a palindrome.