String Summary

151. Reverse Words in a String
    def reverseWords_02(self, s):
        """
        :type s: str
        :rtype: str
        """
        def revS(start, end):
            while start < end:
                s[start], s[end] = s[end], s[start]
                start, end = start + 1, end - 1

        s = s.strip()
        s = list(s)
        space = 0
        count = 0

        for i in range(len(s)):
            if s[i] == " " and space < 1:
                space += 1
                s[count] = s[i]
            elif s[i] == " ":
                continue
            else:
                space = 0
                s[count] = s[i]
            count += 1

        s = list(s[:count])[::-1]
        start = 0
        for i in range(len(s)):
            if s[i] == " ":
                revS(start, i - 1)
                start = i + 1
            elif i == len(s) - 1:
                revS(start, i)

        return ''.join(s)

 125. Valid Palindrome
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        alphanumeric = [chr(i) for i in range(65,91)] + [chr(i).lower() for i in range(65,91)] + [str(i) for i in range(10)]
        start, end = 0, len(s) - 1
        while start < end:
            while start < len(s) and s[start] not in alphanumeric:
                start += 1
            while end >= 0 and s[end] not in alphanumeric:
                end -= 1
            if start >= len(s) and end < 0:
                break
            if s[start].lower() == s[end].lower():
                start += 1
                end -= 1
            else:
                return False
        return True 


171. Excel Sheet Column Number
class Solution(object):
    def titleToNumber(self, s):
        """
        :type s: str
        :rtype: int
        """
        result =  0
        j = 0
       
        for i in range(len(s) - 1, -1, -1):
            result += (ord(s[i]) - 64) * 26 ** j
            j += 1
       
        return result


168. Excel Sheet Column Title
class Solution(object):
    def convertToTitle(self, n):
        """
        :type n: int
        :rtype: str
        """
        res = ""
        d = {}
        for i in range(26):
            d[i] = chr(i+65).upper()
       
        while n:
            cur = (n - 1) % 26
            res = d[cur] + res
            n = (n - 1) / 26

        return res 

17. Letter Combinations of a Phone Number
    def letterCombinations(self, digits):
        """
        :type digits: str
        :rtype: List[str]
        """
        keyMap = {'2': 'abc', '3': 'def', '4': 'ghi', '5': 'jkl', '6': 'mno', '7': 'pqrs', '8': 'tuv', '9': 'wxyz'}
      
        def dfs(pool, temp):
            if len(temp) == len(digits):
                result.append(''.join(temp))
                return
          
            for i in keyMap[pool[0]]:
                dfs(pool[1:], temp + [i])
      
        if not digits:
            return []
      
        result = []
        dfs(digits, [])
        return result

38. Count and Say
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        res = '1'
        for i in xrange(n-1):
            new_res = ''
            j = 0
            while j < len(res):
                count = 1
                while j < len(res)-1 and res[j] == res[j+1]:
                    j = j + 1
                    count = count + 1
                j = j + 1
                new_res = new_res + str(count) + res[j-1]
            res = new_res
        return res