Don't Be a Subsequence

#include <iostream>
#include <string>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
#include <deque>
#include <map>
#define range(i,a,b) for(auto i=a;i<=b;++i)
#define LL long long
#define itrange(i,a,b) for(auto i=a;i!=b;++i)
#define rerange(i,a,b) for(auto i=a;i>=b;--i)
#define fill(arr,tmp) memset(arr,tmp,sizeof(arr))
using namespace std;
int dp[int(2e5+5)],alpha[30],NEXT[int(2e5+5)][30],len;
string word;
void init(){
    cin>>word;
    len=int(word.size());
    range(i,0,25)alpha[i]=len;
    rerange(i,len-1,0){
        alpha[word[i]-‘a‘]=i;
        range(j,0,25)NEXT[i][j]=alpha[j];
        dp[i]=int(1e9+7);
    }
    dp[len]=1;
}
void solve(){
    rerange(i,len-1,0)
    range(j,0,25)dp[i]=min(dp[i],dp[NEXT[i][j]+1]+1);
    int pos=0;
    rerange(i,dp[0],0)range(j,0,25)if(dp[pos]==dp[NEXT[pos][j]+1]+1){
        putchar(‘a‘+j);
        pos=NEXT[pos][j]+1;
        break;
    }
}
int main() {
    init();
    solve();
    return 0;
}