程序代码：

//数字之美：
#include <stdio.h>
#include <memory.h>
#include <string.h>

int main(void)
{
    char s0[100005];
    char s1[100005];
    char s2[100005];
    int num4,num7,numall,halfnumall;
    int i,len,j;
    while(scanf("%s",s0)!=EOF)
    {
        memset(s1,0,sizeof(char)*100005);
        memset(s2,0,sizeof(char)*100005);
        //1.找到最小幸运数字：
        len=strlen(s0);
        //s0=44447777,44474777,....44477774
        //s2=11112222,11121222,....11122221
        //s0=444888,4548,12345678,88
        //s2=111333,1213,11112223,33
        for(i=len-1; i>=0; i--)
        {
            if(s0[i]<='4')
            {
                s2[i+1]=1;
            }
            else if(s0[i]<='7')
            {
                s2[i+1]=2;
            }
            else
            {
                s2[i+1]=3;
            }
        }
        //s2=111333,1213,11112223,33
        //s2=112111,1221,11121111,111
        for(i=len; i>0; i--)
        {
            if(s2[i]>=3)
            {
                s2[i-1]++;
                s2[i]=1;
                for(j=len; j>i; j--)
                {
                    s2[j]=1;
                }
            }
        }
        //s1=447444,4774,44474444,444
        if(s2[0]==0)
        {
            for(i=1; i<=len+3; i++)
            {
                switch(s2[i])
                {
                    case 1:
                        s1[i-1]=4;
                        break;
                    case 2:
                        s1[i-1]=7;
                        break;
                }
            }
        }
        else
        {
            for(i=0; i<=len+3; i++)
            {
                switch(s2[i])
                {
                    case 1:
                        s1[i]=4;
                        break;
                    case 2:
                        s1[i]=7;
                        break;
                }
            }
        }
        //2.将最小幸运数字变为超级幸运数字：
        //2.1统计4,7个数：
        for(i=0,num4=0,num7=0; i<=len+3; i++)
        {
            if(s1[i]==4)
            {
                num4++;
            }
            else if(s1[i]==7)
            {
                num7++;
            }
        }
        numall=num4+num7;
        //2.2 若为奇数：
        if(numall%2)
        {
            numall++;
            halfnumall=numall/2;
            for(i=0; i<halfnumall; i++)
            {
                s1[i]=4;
            }
            for(i=0; i<halfnumall; i++)
            {
                s1[halfnumall+i]=7;
            }
            goto out;
        }
        //2.2 若为偶数：
        while(num7>num4)
        {
            for(i=numall+3; i>=0; i--)
            {
                if(s1[i]==4)
                {
                    s1[i]=7;
                    for(i++; i<=numall+3; i++)
                    {
                        if(s1[i]==7)
                        {
                            s1[i]=4;
                        }
                    }
                    break;
                }
            }
            //统计4,7个数
            for(i=0,num4=0,num7=0; i<=numall+3; i++)
            {
                if(s1[i]==4)
                {
                    num4++;
                }
                else if(s1[i]==7)
                {
                    num7++;
                }
            }
            if(num4==0)//全都是7的话：
            {

                numall+=2;
                halfnumall=numall/2;
                for(i=0; i<halfnumall; i++)
                {
                    s1[i]=4;
                }
                for(i=0; i<halfnumall; i++)
                {
                    s1[halfnumall+i]=7;
                }
                goto out;

            }
        }
        for(i=numall+3; i>=0&&num4>num7; i--)
        {
            if(s1[i]==4)
            {
                s1[i]=7;
                num4--;
                num7++;
            }
        }
out:
        for(i=0;; i++)
        {
            if(s1[i]==4)
            {
                printf("4");
            }
            else if(s1[i]==7)
            {
                printf("7");
            }
            else
            {
                break;
            }
        }
        printf("\n");
    }
    return 0;
}

[此贴子已经被作者于2015-11-9 10:53编辑过]

程序代码：

//数字之美：
#include <stdio.h>
#include <memory.h>
#include <string.h>


int main(void) {
    char s[100005];
    int num4,num7,halflen;
    int i,len,j;
    memset(s,0,sizeof(char)*100005);
    while(scanf("%s",s)!=EOF) {
        len=(int)strlen(s);
        for(i=len-1; i>=0; i--) {
            s[i+1]=s[i];
        }
        s[0]='0';
        len++;
        //1.找到最小幸运数字：
        for(i=1; i<len; i++) {
            if(s[i]>'7') {
                s[i-1]++;
                for(j=i; j<len; j++) {
                    s[j]='0';
                }
                i=0;
            }
        }
        if(s[0]=='1') {
            for(i=0; i<len; i++) {
                s[i]='4';
            }
        } else {
            for(i=1; i<len; i++) {
                if(s[i]<'4') {
                    for(j=i; j<len; j++) {
                        s[j]='4';
                    }
                    break;
                } else if(s[i]=='4') {
                } else if(s[i]<'7') {
                    s[i]='7';
                    for(j=i+1; j<len; j++) {
                        s[j]='4';
                    }
                    break;
                }
            }
            for(i=1; i<len; i++) {
                s[i-1]=s[i];
            }
            s[len-1]=0;
            len--;
        }
        //2.将最小幸运数字变为超级幸运数字：
        //2.1统计4,7个数：
        for(i=0,num4=0,num7=0; i<len; i++) {
            if(s[i]=='4') {
                num4++;
            } else if(s[i]=='7') {
                num7++;
            }
        }
        if(len%2) {
            len++;
            halflen=len/2;
            for(i=0; i<halflen; i++) {
                s[i]='4';
            }
            for(i=0; i<halflen; i++) {
                s[halflen+i]='7';
            }
            s[len]=0;
            printf("%s\n",s);
            goto out1;
        } else {
            while(num7>num4) {
                for(i=len-1; i>=0; i--) {
                    if(s[i]=='4') {
                        s[i]='7';
                        for(j=i+1; j<=len-1; j++) {
                            s[j]='4';
                        }
                        break;
                    }
                }
                //统计4个数
                for(i=0,num4=0,num7=0; i<len; i++) {
                    if(s[i]=='4') {
                        num4++;
                    } else if(s[i]=='7') {
                        num7++;
                    }
                }
                if(num4==0) { //全都是7的话：
                    len+=2;
                    halflen=len/2;
                    for(i=0; i<halflen; i++) {
                        s[i]='4';
                    }
                    for(i=0; i<halflen; i++) {
                        s[halflen+i]='7';
                    }
                    s[len]=0;
                    printf("%s\n",s);
                    goto out1;
                }
            }
        }
        for(i=len-1; num4>num7; i--) {
            if(s[i]=='4') {
                s[i]='7';
                num4--;
                num7++;
            }
        }
        printf("%s\n",s);
    out1:
        ;
        memset(s,0,sizeof(char)*100005);
    }
    return 0;
}


/**************************************************************
    Problem: 1092
    User:

    Language: C
    Result: Accepted
    Time:0 ms
    Memory:1056 kb
****************************************************************/