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

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/*
    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
    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)
    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
*/
#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <map>
using namespace std;

const int MAXN = 1e5 + 10;
const int INF = 0x3f3f3f3f;
char s[MAXN];

int main(void)        //Codeforces Round #288 (Div. 2) B. Anton and currency you all know
{
    #ifndef ONLINE_JUDGE
        freopen ("B.in", "r", stdin);
    #endif

    scanf ("%s", &s);

    int len = strlen (s);
    bool flag = false;
    int x = -1;
    for (int i=0; i<len-1; ++i)
    {
        if ((s[i] - '0') % 2 == 0)
        {
            x = i;
            if (s[i] < s[len-1])
            {
                swap (s[i], s[len-1]);
                flag = true;    break;
            }
        }
    }

    if (!flag)
    {
        swap (s[x], s[len-1]);
    }
    (x != -1) ? printf ("%s
", s) : puts ("-1");

    return 0;
}

if (len == 1)
    {
        puts ("-1");
    }
    else if (len == 2)
    {
        if ((s[0] - '0') % 2 != 0)    puts ("-1");
        else
        {
            printf ("%c%c
", s[1], s[2]);
        }
    }
    else if (len == 3)
    {
        if ((s[1] - '0') % 2 == 0)
        {
            if ((s[0] - '0') % 2 == 0)
            {
                if (s[0] > s[1])    swap (s[0], s[2]);
                else    swap (s[1], s[2]);
                printf ("%s
", s);
            }
            else
            {
                swap (s[1], s[2]);
                printf ("%s
", s);
            }
        }
        else if ((s[0] - '0') % 2 == 0)
        {
            swap (s[0], s[2]);
            printf ("%s
", s);
        }
        else    puts ("-1");
    }
    else
    {
        char ch = '0';    int x = -1;
        for (int i=len-2; i>=0; --i)
        {
            if ((s[i]-'0') % 2 == 0)
            {
                if (ch <= s[i])
                {
                    if (ch == s[i])
                    {
                        if (s[i] < s[len-1])
                        {
                            ch = s[i];     x = i;
                        }
                    }
                    else
                    {
                        ch = s[i];     x = i;
                    }
                }
            }
        }
        if (x == -1)    puts ("-1");
        else
        {
            swap (s[x], s[len-1]);
            printf ("%s
", s);
        }
    }