17. 交替字符串

如果字符串str3能够由str1和str2中的字符按顺序交替形成，那么称str3为str1和str2的交替字符串。例如str1="abc"，str2="def"，那么"adbecf", "abcdef", "abdecf", "abcdef", "adefbc"等等都为str1和str2的交替字符串。更形式化的，str3的生成算法如下：

str3=""
while str1不为空 or str2不为空:
  把str1或str2的首字符加入到str3,并从str1或str2中删除相应的字符
end

给定str1, str2,和str3，判断str3是否为str1和str2的交替字符串。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication14
{
    class Program
    {
        static void Main(string[] args)
        {
            string str1 = "abc";
            string str2 = "def";
            string[] str3 = { "adbecf","abcdef","abdecf","abcdef","adefbc"};
            foreach (string str in str3)
            {
                Console.WriteLine("string 1 is {0}, string 2 is {1}, string 3 is {2}, result:{3}",str1,str2,str,IsAlternatelyString(str1,str2,str));
            }
        }

        static bool IsAlternatelyString(string str1, string str2, string str3)
        {
            int i = 0;
            int j = 0;
            int k = 0;
            while (k<str3.Length&&i<str1.Length&&j<str2.Length)
            {
                if (str1[i] == str3[k])
                {
                    i++;
                    k++;
                }
                else
                {
                    if (str2[j] == str3[k])
                    {
                        j++;
                        k++;
                    }
                    else
                    {
                        return false;
                    }
                }
            }
            while (k<str3.Length&&i<str1.Length)
            {
                if (str1[i] == str3[k])
                {
                    i++;
                    k++;
                }
                else
                {
                    return false;
                }
            }
            while (k<str3.Length&&j<str2.Length)
            {
                if (str2[j] == str3[k])
                    {
                        j++;
                        k++;
                    }
                    else
                    {
                        return false;
                    }
            }
            return true;
        }
    }
}