leetcode:125. Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

解題思路：

在字符串前後各設一個指針，每次指向的先判別是否字母或數字，如果是小寫字母則轉爲大寫字母，再判別當前兩個指針所指向的字符是否一樣，若不一樣則不爲迴文串，跳出比較，若一樣則前後指針再查找下一組字符，終止條件爲前指針>=後指針。

class Solution {
public:
    bool isPalindrome(string s) {
        int len = s.length();
        int i = 0, j = len - 1;
        bool flag = 1;
        while(i < j){
            while(i < j){
                if(s[i] >= 'A' && s[i] <= 'Z')break;
                if(s[i] >= '0' && s[i] <= '9')break;
                if(s[i] >= 'a' && s[i] <= 'z'){
                    s[i] -= 32;
                    break;
                }
                i++;
            }
            while(j > i){
                if(s[j] >= 'A' && s[j] <= 'Z')break;
                if(s[j] >= '0' && s[j] <= '9')break;
                if(s[j] >= 'a' && s[j] <= 'z'){
                    s[j] -= 32;
                    break;
                }
                j--;
            }
            if(s[i] != s[j]){flag = 0; break;}
            i++,j--;
        }
        return flag;
    }
};