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