(模板)c++ 大数（正数加减乘除）

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char ip[mxn];struct bign {    int len, dig[mxn];    bign() {        len = 0, dig[0] = 0;    }bool input() {        if(scanf("%s", ip) == EOF) return 0;        if(strcmp(ip, "0") == 0) {            len = 0, dig[0] = 0;            return 1;        }        int n = strlen(ip);        for( int i = 0; i < n; ++i )            dig[i] = ip[n-i-1] - '0';        len = n - 1;        return 1;    }    void output() {        for(int i = len; i >= 0; --i)            printf("%d", dig[i]);    }    bign operator + (const bign &b) const {        int i, carry;        bign c;        for(i = carry = 0; i <= len || i <= b.len || carry; ++i) {            if(i <= len) carry += dig[i];            if(i <= b.len) carry += b.dig[i];            c.dig[i] = carry % 10;carry /= 10;        }        c.len = i - 1;        return c;    }    bign operator + (const int &k) const {        int i, b = k;        bign c;        for(i = 0; i <= len || b; ++i) {            if(i <= len) b += dig[i];            c.dig[i] = b % 10;            b /= 10;        }        c.len = i - 1;        return c;    }    bign operator * (const int &k) const {        int i, carry, b = k;        bign c;        for(i = carry = 0;i <= len || carry; ++i) {            if( i <= len ) carry += b * dig[i];            c.dig[i] = carry % 10;            carry /= 10;        }        i--;        while(i && !c.dig[i]) --i;        c.len = i;        return c;    }    bign operator * (const bign &b) const {        int i, j, carry;        bign c;        for(i = len + b.len + 1; i >= 0; -- i) c.dig[i] = 0;        for(i = 0; i <= len; ++i) {            carry = 0;            for(j = 0; j <= b.len || carry; ++j) {                carry += c.dig[i+j];                if(j <= b.len) carry += dig[i] * b.dig[j];                c.dig[i+j] = carry % 10;                carry /= 10;            }        }        i = len + b.len + 1;        while(i && !c.dig[i]) --i;        c.len = i;        return c;    }};//c为商， d为余数void div(bign a, int b, bign &c, int &d) {    int i;    for(i = a.len, d = 0; i >= 0; --i) {        d = d * 10 + a.dig[i];        c.dig[i] = d / b;        d = d % b;    }    i = a.len;    while( i && !c.dig[i] ) -- i;    c.len = i;}
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