KMP算法备忘

今天学习了一下串的模式匹配算法,在这里记一下,以后好参考.具体的原理就不说了,可以参考严蔚敏老师的<<数据结构>>一书第79页到第84页,那里讲得比较深.我用C#写了一下这个算法,并且实现为string类的扩展函数.代码如下:

 

    using System;

    class KMPClass
    {
        static void Main(string[] args)
        {
            string mother = "concatenation";
            string child = "cat";
            int k = mother.indexKMP(child);
            Console.WriteLine(k);
            Console.Read();
        }
    }

    public static class MyClass
    {
        /**//// <summary>
        /// 简单版本，效率低
        /// </summary>
        /// <param name="str"></param>
        /// <param name="child"></param>
        /// <returns></returns>
        public static int index(this string str,string child)
        {
            int i = 0;
            int j = 0;
            while (i < str.Length  && j < child.Length )
            {
                if (str[i] == child[j])
                {
                    i++;
                    j++;
                }
                else
                {
                    i = i - j + 1;
                    j = 0;
                }

               
            }
            if (j >child.Length - 1)
            {
                return i - child.Length;
            }
            else
            {
                return -1;
            }

        }

        /**//// <summary>
        /// KMP算法
        /// </summary>
        /// <param name="str"></param>
        /// <param name="pattern"></param>
        /// <returns></returns>
        public static int indexKMP(this string str, string pattern)
        {
            int i = 0;
            int j = 0;
            int[] nextVal = GetNextVal(pattern);

            while (i<str.Length && j<pattern.Length)
            {
                if (j == -1 || str[i] == pattern[j])
                {
                    i++;
                    j++;
                }
                else
                {
                    j = nextVal[j];
                }
            }

            if (j > pattern.Length - 1)
            {
                return i - pattern.Length;
            }
            else
            {
                return -1;
            }
        }

        private static int[] GetNextVal(string pattern)
        {
            int j = 0, k = -1;
            int[] nextVal = new int[pattern.Length];

            nextVal[0] = -1;

            while (j < pattern.Length - 1)
            {
                if (k == -1 || pattern[j] == pattern[k])
                {
                    j++;
                    k++;
                    if (pattern[j] != pattern[k])
                    {
                        nextVal[j] = k;
                    }
                    else
                    {
                        nextVal[j] = nextVal[k];
                    }
                }
                else
                {
                    k = nextVal[k];
                }
            }

            return nextVal;
        }
    }

KMP算法C#代码

                                                                                                             2009年2月8日