(C#习题) 字符串

(C#习题) 字符串

1.输入一个英文句子，翻转句子中单词的顺序，但单词内字符的顺序不变。句子中单词以空格符隔开。为简单起见，标点符号和普通字母一样处理。
例如输入“I am a student.”，则输出“student. a am I”。

namespace ConsoleApplication2
{
    class Program
    {
        // Use char array.
        static string Reverse1(string input)
        {
            char[] chars = input.ToCharArray(); 
            char tmp;
            int indexStart = 0;
            int indexEnd = chars.Length - 1; 
            // Reverse all chars. 
            for (int i = indexStart, j = indexEnd; i < j; i++, j--)
            {
                tmp = chars[i];
                chars[i] = chars[j];
                chars[j] = tmp;
            }
            
            // Reverse sub strings split by blankspace.
            indexStart = 0;
            for (int k = 0; k < chars.Length; k++)
            {
                if (chars[k].Equals(' '))
                {
                    // Reverse substirngs.
                    indexEnd = k - 1; 
                    for (int m = indexStart, n = indexEnd ; m<n; m++, n--)
                    {
                        tmp = chars[m];
                        chars[m] = chars [n];
                        chars[n] = tmp; 
                    }
                    indexStart = k + 1; 
                }
            }
            //Convert to string from char array. 
            string reservedString = new string(chars);

            return reservedString;
        }

        // Use string method in C#.   Split -> Inverted order output substrings.
        static string Reverse2(string input)
        {
            // Split by blankspace.
            string[] subStrings = input.Split(' '); 
            StringBuilder stringBuilder = new StringBuilder(input.Length); 
            // Order output subString; 
            for (int i = subStrings.Length- 1; i>=0 ; i--)
            {
                if (i == 0)
                {
                    stringBuilder.Append(subStrings[i]);
                }
                else
                {
                    stringBuilder.Append(subStrings[i]).Append(' ');
                }
            }
            return stringBuilder.ToString(); 
        }

        // Use string method in C#.   Reverse -> Split -> Reverse
        static string Reverse3(string input)
        {
            char[] chars = input.ToCharArray();
            Array.Reverse(chars);
            string reversedString = new string(chars);

            string[] subStrings = reversedString.Split(' ');
            StringBuilder stringBuilder = new StringBuilder(input.Length); 
            for (int i = 0; i < subStrings.Length ; i++)
            {
                chars = subStrings[i].ToCharArray();
                Array.Reverse(chars);
                reversedString = new string(chars);
                if (i == subStrings.Length - 1)
                {
                    stringBuilder.Append(reversedString);
                }
                else
                {
                    stringBuilder.Append(reversedString).Append(' ');
                }
            }
            return stringBuilder.ToString(); 
        }

        // Use linq 
        static string Reverse4(string input)
        {
            string output = new string(input.ToCharArray().Reverse().ToArray());
            string[] subStrings = output.Split(' ');

            StringBuilder stringBuilder = new StringBuilder(input.Length);
            string reversedString;
            for (int i = 0; i < subStrings.Length; i++)
            {
                reversedString = new string(subStrings[i].ToCharArray().Reverse().ToArray());
                if (i == subStrings.Length -1)
                {
                    stringBuilder.Append(reversedString);
                }
                else
                {
                    stringBuilder.Append(reversedString).Append(' ');
                }
            }
            return stringBuilder.ToString();
        }

        static void Main(string[] args)
        {
            string myString = "I am a student.";
            Console.WriteLine("Reversed sentence is: {0}", Reverse1(myString));
            Console.WriteLine("Reversed sentence is: {0}", Reverse2(myString));
            Console.WriteLine("Reversed sentence is: {0}", Reverse3(myString));
            Console.WriteLine("Reversed sentence is: {0}", Reverse4(myString));  
            Console.ReadLine();
        }
    }
}

题目：在一个字符串中找到第一个只出现一次的字符。如输入abaccdeff，则输出b。

namespace ConsoleApplication2
{
    class Program
    {
        // Search in array .  o(n2)
        static char FirstShowOneTimeCharacter1(string input)
        {
            int[] showTimes = new int[input.Length]; 
            char[] chars = input.ToCharArray();
            int index = 0; 
            for (;index < input.Length; index++)
            {
                for (int i = 0; i < input.Length; i++)
                {
                    if (chars[index].Equals(chars[i]))
                    {
                        showTimes[index]++;
                    }
                }
                if (showTimes[index] == 1)
                { 
                    break;
                }
            }
            return chars[index]; 
        }

        // Use hash table.  O(n2) . It is wrong here.  
        static char FirstShowOneTimeCharacter2(string input)
        {
            Hashtable hashtable = new Hashtable(256);       //Char : 1 byte. 
            char[] chars = input.ToCharArray();
            for (int i = 0; i < input.Length; i++)
            {
                if (hashtable.ContainsKey(chars[i]))   
                {
                    hashtable[chars[i]] = int.Parse(hashtable[chars[i]].ToString()) + 1;
                }
                else
                {
                    hashtable.Add(chars[i], 1);
                }
            }

            // It is reversed output when the result is output. 
            foreach (DictionaryEntry de in hashtable)
            {
                if (de.Value.ToString().Equals("1"))
                {
                    return (char)de.Key;
                }

            }
            return '0'; 
       }

        // Use bitMap. O(n) , array[asciiCode] = showTimes
        static char FirstShowOneTimeCharacter3(string input)
        {
            int[] bitMap = new int[256];
            
            // Set initial show times to zero. 
            for (int i = 0; i < bitMap.Length; i++)
            {
                bitMap[i] = 0;
            }

            // Get all characters' show times. 
            char[] chars = input.ToCharArray();
            for (int i = 0; i < chars.Length; i++)
            {
                // Use (int) to transfer character to its ASCII code.
                bitMap[(int)chars[i]]++;        
            }

            // Get the first character which shows only one time. 
            for (int asciiCode = 0; asciiCode < bitMap.Length; asciiCode++)
            {
                if (bitMap[asciiCode] == 1)
                {
                    return (char)asciiCode;
                }
            }
            return '0'; 
        }

        // Use linq O(?) 
        static char FirstShowOneTimeCharacter4(string input)
        {
            var result = (from item in input group item by item into groups where groups.Count() == 1 select groups).First();
            return result.Key;
        }

        static void Main(string[] args)
        {
            string myString = "abaccdeff";
            Console.WriteLine("First show 1 time's character is: {0}", FirstShowOneTimeCharacter1(myString));   // b
            Console.WriteLine("First show 1 time's character is: {0}", FirstShowOneTimeCharacter2(myString));   // e
            Console.WriteLine("First show 1 time's character is: {0}", FirstShowOneTimeCharacter3(myString));   // b
            Console.WriteLine("First show 1 time's character is: {0}", FirstShowOneTimeCharacter4(myString));   // b
            Console.ReadLine();
        }
    }
}