String

    public String longestPalindrome(String s) {
        int n = s.length();
        if (n < 2) return s;
        String res = "";
        for (int i = 0; i < n * 2 - 1; i++)
        {
            int l = i / 2, r = i / 2;
            if (i % 2 == 1)
            {
                r++;
            }
            String cur = longest(s,l,r);
            if (res.length() < cur.length())
            {
                res = cur;
            }
        }
        return res;
    }
    public String longest(String s, int l, int r)
    {
        while (l >=0 && r < s.length() && s.charAt(l) == s.charAt(r))
        {
            l--;r++;
        }
        return s.substring(l+1,r);
    }

    public String convert(String s, int numRows) {
        char[] c = s.toCharArray();
        int len = c.length;
        StringBuffer[] sb = new StringBuffer[numRows];
        for (int i = 0; i < numRows; i++)
        {
            sb[i] = new StringBuffer();
        }
        int i = 0;
        while (i < len)
        {
            for (int ind = 0; ind < numRows && i < len; ind++)
            {
                sb[ind].append(c[i++]);
            }
            for (int ind = numRows - 2; ind >= 1 && i < len; ind--)
            {
                sb[ind].append(c[i++]);
            }
        }
        for (int in = 1; in < numRows; in++)
        {
            sb[0].append(sb[in]);
        }
        return sb[0].toString();
    }

public boolean isMatch(String s, String p) {

    if (s == null || p == null) {
        return false;
    }
    boolean[][] dp = new boolean[s.length()+1][p.length()+1];
    dp[0][0] = true;
    for (int i = 0; i < p.length(); i++)
    {
        if (p.charAt(i) == ‘*‘ && dp[0][i - 1])
        {
            dp[0][i + 1] = true;
        }
    }
    for (int i = 0; i < s.length(); i++)
    {
        for (int j = 0; j < p.length(); j++)
        {
            if (p.charAt(j) == ‘.‘)
            {
                dp[i+1][j + 1] = dp[i][j];
            }
            if (p.charAt(j) == s.charAt(i))
            {
                dp[i+1][j + 1] = dp[i][j];
            }
            if (p.charAt(j) == ‘*‘)
            {
                if (p.charAt(j - 1) != s.charAt(i) && p.charAt(j - 1) != ‘.‘)
                {
                    dp[i+1][j+1]=dp[i+1][j-1];
                }
                else
                {
                    dp[i+1][j+1]=(dp[i+1][j] || dp[i][j+1] || dp[i+1][j-1]);
                }
            }
        }
    }
     return dp[s.length()][p.length()];
}

    public String longestCommonPrefix(String[] strs) {
    if(strs == null || strs.length == 0)    return "";
    String pre = strs[0];
    int i = 1;
    while(i < strs.length){
        while(strs[i].indexOf(pre) != 0)
            pre = pre.substring(0,pre.length()-1);
        i++;
    }
    return pre;
    }