A typical DP water question-the longest common subsequence POJ 2250

I did not write this article because I think there are too few materials for the longest public subsequence,

It was such a familiar problem that I finally found the bug after WA all morning,

Lessons learned from the morning's pain: as a programmer, even if you are familiar with simple problems, you must keep strict logic at all times.

Let's start with the code with a bug.

#include<cstdio>#include<cstring>#include<iostream>#include<algorithm>#define N 200using namespace std;string sa[N],sb[N],str;int dp[N][N],ans[N];struct node{    int pa,pb;}pre[N][N];int main(){    //freopen("8.in","r",stdin);    while(cin>>str) {        int la=0;        int lb=0;        sa[++la]=str;        while(cin>>str) {            if(str[0]=='#') break;            sa[++la]=str;        }        while(cin>>str) {            if(str[0]=='#') break;            sb[++lb]=str;        }        memset(dp,0,sizeof(dp));        for(int i=1;i<=la;i++) {            for(int j=1;j<=lb;j++) {                int a=dp[i-1][j];                int b=dp[i][j-1];                int inc=0;                if(sa[i]==sb[j]) inc=1;                int c=dp[i-1][j-1]+inc;                if(a>b && a>c) {                    dp[i][j]=a;                    pre[i][j].pa=i-1;                    pre[i][j].pb=j;                }                else if(b>a && b>c) {                    dp[i][j]=b;                    pre[i][j].pa=i;                    pre[i][j].pb=j-1;                }                else {                    dp[i][j]=c;                    pre[i][j].pa=i-1;                    pre[i][j].pb=j-1;                }            }        }        int p1,p2,cnt=0;        for(p1=la,p2=lb;p1>0 && p2>0;){            int np1=pre[p1][p2].pa;            int np2=pre[p1][p2].pb;            if(sa[p1]==sb[p2]) ans[cnt++]=p1;            p1=np1;            p2=np2;        }        //cout<<dp[la][lb]<<endl;        sort(ans,ans+cnt);        for(int i=0;i<cnt-1;i++) cout<<sa[ans[i]]<<" ";        cout<<sa[ans[cnt-1]]<<endl;    }    return 0;}

AC code:

# Include <cstdio> # include <cstring> # include <iostream> # include <algorithm> # define N 200 using namespace std; string sa [N], sb [N], str; int dp [N] [N], ans [N]; struct node {int pa, pb;} pre [N] [N]; int main () {// freopen ("8.in"," r ", stdin); while (cin> str) {int la = 0; int lb = 0; sa [++ la] = str; while (cin> str) {if (str [0] = '#') break; sa [++ la] = str;} while (cin> str) {if (str [0] = '#') break; sb [++ lb] = str;} memset (dp, 0, sizeof (dp); for (int I = 1; I <= la; I ++) {for (int j = 1; j <= lb; j ++) {int a = dp [I-1] [j]; int B = dp [I] [J-1]; int inc = 0; if (sa [I] = sb [j]) inc = 1; int c = dp [I-1] [J-1] + inc; // The problem lies in the following if-else statements: if (c> a & c> B) {dp [I] [j] = c; pre [I] [j]. pa = I-1; pre [I] [j]. pb = J-1;} else if (B> a & B> c) {dp [I] [j] = B; pre [I] [j]. pa = I; pre [I] [j]. pb = J-1;} else {dp [I] [j] = a; pre [I] [j]. pa = I-1; pre [I] [j]. pb = j ;}} int p1, p2, cnt = 0; for (p1 = la, p2 = lb; p1> 0 & p2> 0 ;) {int np1 = pre [p1] [p2]. pa; int np2 = pre [p1] [p2]. pb; if (sa [p1] = sb [p2] & p1> np1 & p2> np2) ans [cnt ++] = p1; p1 = np1; p2 = np2;} sort (ans, ans + cnt); for (int I = 0; I <cnt-1; I ++) cout <sa [ans [I] <"; cout <sa [ans [cnt-1] <endl;} return 0 ;}