贪心 Codeforces Round #288 (Div. 2) B. Anton and currency you all know

 

题目传送门

 1 /*  2  题意：从前面找一个数字和末尾数字调换使得变成偶数且为最大  3  贪心：考虑两种情况：1. 有偶数且比末尾数字大(flag标记)；2. 有偶数但都比末尾数字小(x位置标记)  4  仿照别人写的，再看自己的代码发现有清晰的思维是多重要  5 */  6 #include <cstdio>  7 #include <iostream>  8 #include <algorithm>  9 #include <cmath>  10 #include <cstring>  11 #include <map>  12 using namespace std;  13  14 const int MAXN = 1e5 + 10;  15 const int INF = 0x3f3f3f3f;  16 char s[MAXN];  17  18 int main(void) //Codeforces Round #288 (Div. 2) B. Anton and currency you all know  19 {  20  #ifndef ONLINE_JUDGE  21 freopen ("B.in", "r", stdin);  22 #endif  23  24 scanf ("%s", &s);  25  26 int len = strlen (s);  27 bool flag = false;  28 int x = -1;  29 for (int i=0; i<len-1; ++i)  30  {  31 if ((s[i] - '0') % 2 == 0)  32  {  33 x = i;  34 if (s[i] < s[len-1])  35  {  36 swap (s[i], s[len-1]);  37 flag = true; break;  38  }  39  }  40  }  41  42 if (!flag)  43  {  44 swap (s[x], s[len-1]);  45  }  46 (x != -1) ? printf ("%s\n", s) : puts ("-1");  47  48 return 0;  49 }  50 

 

 1 if (len == 1)  2  {  3         puts ("-1");  4  }  5     else if (len == 2)  6  {  7         if ((s[0] - '0') % 2 != 0)    puts ("-1");  8         else

 9  { 10             printf ("%c%c\n", s[1], s[2]); 11  } 12  } 13     else if (len == 3) 14  { 15         if ((s[1] - '0') % 2 == 0) 16  { 17             if ((s[0] - '0') % 2 == 0) 18  { 19                 if (s[0] > s[1])    swap (s[0], s[2]); 20                 else    swap (s[1], s[2]); 21                 printf ("%s\n", s); 22  } 23             else

24  { 25                 swap (s[1], s[2]); 26                 printf ("%s\n", s); 27  } 28  } 29         else if ((s[0] - '0') % 2 == 0) 30  { 31             swap (s[0], s[2]); 32             printf ("%s\n", s); 33  } 34         else    puts ("-1"); 35  } 36     else

37  { 38         char ch = '0';    int x = -1; 39         for (int i=len-2; i>=0; --i) 40  { 41             if ((s[i]-'0') % 2 == 0) 42  { 43                 if (ch <= s[i]) 44  { 45                     if (ch == s[i]) 46  { 47                         if (s[i] < s[len-1]) 48  { 49                             ch = s[i];     x = i; 50  } 51  } 52                     else

53  { 54                         ch = s[i];     x = i; 55  } 56  } 57  } 58  } 59         if (x == -1)    puts ("-1"); 60         else

61  { 62             swap (s[x], s[len-1]); 63             printf ("%s\n", s); 64  } 65     }

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