poj 3069(贪心算法之字典序排序）

Best Cow Line

POJ - 3617                                                    

FJ is about to take his N (1 ≤ N ≤ 2,000) cows to the annual"Farmer of the Year" competition. In this contest every farmer arranges his cows in a line and herds them past the judges.

The contest organizers adopted a new registration scheme this year: simply register the initial letter of every cow in the order they will appear (i.e., If FJ takes Bessie, Sylvia, and Dora in that order he just registers BSD). After the registration phase ends, every group is judged in increasing lexicographic order according to the string of the initials of the cows' names.

FJ is very busy this year and has to hurry back to his farm, so he wants to be judged as early as possible. He decides to rearrange his cows, who have already lined up, before registering them.

FJ marks a location for a new line of the competing cows. He then proceeds to marshal the cows from the old line to the new one by repeatedly sending either the first or last cow in the (remainder of the) original line to the end of the new line. When he's finished, FJ takes his cows for registration in this new order.

Given the initial order of his cows, determine the least lexicographic string of initials he can make this way.

Input

* Line 1: A single integer: N
* Lines 2..N+1: Line i+1 contains a single initial ('A'..'Z') of the cow in theith position in the original line

Output

The least lexicographic string he can make. Every line (except perhaps the last one) contains the initials of 80 cows ('A'..'Z') in the new line.

Sample Input

6ACDBCB

Sample Output

ABCBCD

字典序排序比较类问题经常用得上贪心算法。

错解：贪心原则：左右相比，取较小字母。

#include <iostream>#include <cstring>#include <cstdio>#include <algorithm>using namespace std;int main(){    char s[2100];    int n;    scanf("%d", &n);    for(int i = 1; i <= n; i ++)        cin>>s[i];        int cnt = 0;        int x = 1, y = n;        char t[2100];    while(cnt < n)    {        if(s[x] < s[y])            {                t[cnt] = s[x];                x ++;            }        else            {                t[cnt] = s[y];                y --;            }            cnt ++;    }    cout<<t<<endl;    return 0;}

正解：遵循原则：左右最早找到最小字母。因为如果按照错解的思路，遇到前后相等的字母就不合适了。正解遇到相同字母，则找最早出现最小字母。

#include <iostream>#include <cstring>#include <cstdio>#include <algorithm>using namespace std;int main(){    char s[2100];    int n;    int cnt = 0;    scanf("%d", &n);    for(int i = 0; i < n; i ++)        cin>>s[i];    int a = 0,b = n - 1;    while(a <= b)    {        bool left = false;        for(int i = 0; a + i <= b; i ++)        {            if(s[a + i] < s[b - i])            {                left = true;                break;            }            else if(s[a + i] > s[b - i])            {                left = false;                break;            }        }        if(left)        putchar(s[a++]);    else        putchar(s[b--]);        cnt ++;        if(cnt % 80 == 0)        {            printf("\n");        }    }    printf("\n");    return 0;}