用StrAssign生成S1=’THIS IS A BOOK’，S2=’S’，S3=’ESE ARE’ 用SubString(S,S1,3,7)求出S=’IS IS A’ 用ConCat(T,S1,S2)求出T=’THIS IS A BOOKS’ 用Replace(T,S,S3)求出T=’THESE ARE BOOKS’（选做） 2. 用StrAssign生成U=’XYXYXYXYXYXY’，W=’w’ 用SubString(V,U,6,3)求出V=’YXY’ 用Replace(U,V,W)求出U=’XWXWXW’编写程序

好的，下面是一个用C语言实现的程序，可以完成这两个问题的要求：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define OK 1
#define ERROR 0
#define OVERFLOW -1

typedef struct {
    char *ch;
    int length;
} HString;

int StrAssign(HString *t, char *chars) {
    /*生成一个其值等于串常量chars的串T*/
    int i, j;
    char *c;
    if (t->ch) free(t->ch);/*释放T原有空间*/
    for (i = 0, c = chars; c[i] != '\0'; i++);/*求chars的长度*/
    if (!i) {
        t->ch = NULL;
        t->length = 0;
    } else {
        t->ch = (char *) malloc(i * sizeof(char));
        if (!(t->ch))
            exit(OVERFLOW);
        for (j = 0; j < i; j++)
            t->ch[j] = chars[j];
        t->length = i;
    }
    return OK;
}

int SubString(HString *t, HString s, int pos, int len) {
    /*用t返回串s的第pos个字符起长度为len的子串*/
    int i;
    if (pos < 1 || pos > s.length || len < 0 || pos + len - 1 > s.length)
        return ERROR;
    if (t->ch) free(t->ch);/*释放t原有空间*/
    if (!len) {
        t->ch = NULL;
        t->length = 0;
    } else {
        t->ch = (char *) malloc(len * sizeof(char));
        if (!(t->ch))
            exit(OVERFLOW);
        for (i = 0; i < len; i++)
            t->ch[i] = s.ch[pos - 1 + i];
        t->length = len;
    }
    return OK;
}

int ConCat(HString *t, HString s1, HString s2) {
    /*用t返回由s1和s2联接而成的新串*/
    int i;
    if (t->ch) free(t->ch);/*释放t原有空间*/
    t->ch = (char *) malloc((s1.length + s2.length) * sizeof(char));
    if (!(t->ch))
        exit(OVERFLOW);
    for (i = 0; i < s1.length; i++)
        t->ch[i] = s1.ch[i];
    for (i = 0; i