两个字符串，A和B，从A变成B，可以通过增加、删除、修改字母来实现，其中每增加、删除、修改一个字母为一步，此程序可以通过动态规划的方法计算出字符串编辑的最少步骤数(Two strings, A and B, from A to B, can be achieved by adding, deletion and modification of letters, each of which increase, delete and modify a letter into one step, this program can be obtained by the dynamic programming method to calculate the minimum number of steps of string edit)