Go: package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"strconv"
)

const (
	initialBufSize = 10000000
	maxBufSize     = 100000000
)

var sc *bufio.Scanner

func initScanner(r io.Reader) *bufio.Scanner {
	buf := make([]byte, initialBufSize)

	sc := bufio.NewScanner(r)
	sc.Buffer(buf, maxBufSize)
	sc.Split(bufio.ScanWords) // bufio.ScanLines
	return sc
}

func main() {
	sc = initScanner(os.Stdin)
	resolve()
}

func nextInt(sc *bufio.Scanner) int {
	sc.Scan()
	a, _ := strconv.Atoi(sc.Text())
	return int(a)
}
func nextString(sc *bufio.Scanner) string {
	sc.Scan()
	return sc.Text()
}

func nextIntSlice(sc *bufio.Scanner, n int) (a []int) {
	a = make([]int, n)
	for i := 0; i < n; i++ {
		a[i] = nextInt(sc)
	}
	return a
}

var dp [][]int

func initDP(n int) {
	dp = make([][]int, n+1)
	for i := 0; i < len(dp); i++ {
		dp[i] = make([]int, n+1)
		for j := 0; j < len(dp[0]); j++ {
			dp[i][j] = -1
		}
	}
}

func resolve() {
	ans := []int{}
	for true {
		n := nextInt(sc)
		if n == 0 {
			break
		}
		as := nextIntSlice(sc, n)
		initDP(n)
		_rec(as, n, 0, n)
		ans = append(ans, dp[0][n])
	}
	for _, v := range ans {
		fmt.Println(v)
	}
	return
}

func _rec(as []int, n, l, r int) int {
	if dp[l][r] > -1 {
		return dp[l][r]
	}
	if r-l == 1 { // prevent to hit daruma
		dp[l][r] = 0
		return dp[l][r]
	}
	if r-l == 2 {
		if abs(as[l]-as[r-1]) <= 1 {
			dp[l][r] = 2
		} else {
			dp[l][r] = 0
		}
		return dp[l][r]
	}
	if abs(as[l]-as[r-1]) <= 1 {
		v := _rec(as, n, l+1, r-1)
		// if we can remove internal blocks
		if (r - 1 - (l + 1)) == v {
			dp[l][r] = v + 2
			return dp[l][r]
		} else {
			dp[l][r] = v
		}
	}
	for i := l + 1; i < r; i++ {
		v := _rec(as, n, l, i) + _rec(as, n, i, r)
		if dp[l][r] < v {
			dp[l][r] = v
		}
	}
	return dp[l][r]
}

func abs(v int) int {
	if v < 0 {
		return -v
	}
	return v
}

C++:  #include<bits/stdc++.h>
 using namespace std;
#define INF 1000000000
#define REP(i,n) for(int (i)=0;(i)<(int)(n);(i)++)
typedef long long LL;
int N;
bool ok[1001][1001];
int dp[1001][1001];
int ans[1001];
int main(){
    while(cin>>N,N){
       vector<int>v(N);
       REP(i,N)cin>>v[i];
       REP(i,1001)REP(j,1001){ok[i][j]=false;dp[i][j]=0;}
       //ok????????????
       /*REP(i,N){
           for(int j=i+1;j<N;j++){
               if(j-i==1&&abs(v[i]-v[j])<=1){ok[i][j]=true;dp[i][j]=2;continue;}
               if(j-i==2){ok[i][j]=false;dp[i][j]=0;continue;}
               for(int k=i+1;k+1<j;k++){
                   if(abs(v[i]-v[j])>1)continue;
                   if(ok[i][k]==false)continue;
                   if(ok[k+1][j]==false)continue;
                   ok[i][j]=true;
                   dp[i][j]=j-i+1;
               }
           }
       }*/
       for(int k=1;k<=N-1;k++){
          REP(i,N){
              if(i+k>=N)continue;
              if(k==1&&abs(v[i]-v[i+1])<=1){ok[i][i+1]=true;dp[i][i+1]=2;continue;}
              if(k==1)continue;
              if(k==2){ok[i][i+2]=false;dp[i][i+2]=0;continue;}
              if(abs(v[i]-v[i+k])<=1&&ok[i+1][i+k-1]==true){ok[i][i+k]=true;dp[i][i+k]=k+1;}
              for(int j=1;j<=k;j++){
                  if(ok[i][i+j]&&ok[i+j+1][i+k]){ok[i][i+k]=true;dp[i][i+k]=k+1;}
                  /*if(abs(v[i]-v[i+j]>1))continue;
                  if(ok[i][i+j-1]==false)continue;
                  if(ok[i+j+1][i+k]==false)continue;
                  ok[i][i+k]=true;
                  dp[i][i+k]=k+1;*/
              }
          }
       }
       REP(i,1001){
           ans[i]=0;
       }
       for(int i=1;i<N;i++){
           ans[i]=max(ans[i],ans[i-1]);
           ans[i]=max(ans[i],dp[0][i]);
           for(int j=1;j<i;j++){
               ans[i]=max(ans[i],ans[j]+dp[j+1][i]);
           }
       }
       //REP(i,N){REP(j,N)cout<<dp[i][j]<<" ";cout<<endl;}
       //REP(i,N)cout<<ans[i]<<endl;
       cout<<ans[N-1]<<endl;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
#define vi vector<int>
#define vvi vector<vector<int> >
template<typename Head, typename... Tail> auto vectors(Head x, Tail... tail) { auto inner = vectors(tail...); return vectors<decltype(inner)>(x, inner); }
using ll = long long;
using ld =long double;
#define int ll
#define INF 1e9
#define EPS 0.0000000001
#define rep(i,n) for(int i=0;i<n;i++)
#define loop(i,s,n) for(int i=s;i<n;i++)
#define all(in) in.begin(), in.end()
template<class T, class S> void cmin(T &a, const S &b) { if (a > b)a = b; }
template<class T, class S> void cmax(T &a, const S &b) { if (a < b)a = b; }
#define MAX 9999999
int dx[]={0,0,1,-1};
int dy[]={1,-1,0,0};
int W,H;
bool inrange(int x,int y){return (0<=x&&x<W)&&(0<=y&&y<H);}
using namespace std;
typedef pair<int, int> pii;
typedef pair<int,pii> piii;
#define MP make_pair
signed main(){
    int n;
    while(cin >> n,n){
        vector<vector<int>>dp(n,vector<int>(n,0));
        vector<int>array(n);
        rep(i,n) cin >> array[i];
        rep(k,n)rep(i,n-1){
            if(!k || i+k>n-1)continue;
            if(k == 1){
                if(abs(array[i]-array[i+1]) <= 1) dp[i][i+k] = 1;
            } else{
                if((abs(array[i]-array[i+k])<=1 && dp[i+1][i+k-1])){
                    cmax(dp[i][i+k],dp[i+1][i+k-1]+2);
                }
                for(int j = i+1; j<i+k-1;j++){
                    if(dp[i][j] && dp[j+1][i+k])cmax(dp[i][i+k],dp[i][j] + dp[j+1][i+k]);
                }
            }
        }
        int ans = 0;
        for (int s = 0;s < n;s++){
            int temp = 0;
            for(int i = s; i<n;i++){
                for(int j = n-1; j >= 0;--j){
                    if(dp[i][j]){
                        temp += j-i+1;
                        i = j;
                    }
                }
                cmax(ans, temp);
            }
        }
        cout << ans << endl;
    }
}

C++: #include<bits/stdc++.h>
using namespace std;
#define int long long 

int n; 
int w[310];
int memo[310][310];

int dfs(int l, int r){ 
  
  if( memo[l][r] != -1 ){ return memo[l][r];}
 
  int len = r-l+1;
  if( len <= 2 ){ 
    if( len == 1 ){ return 0;}
    if( len == 2 ){ 
      if(abs(w[l]-w[r]) <= 1 ){ return 2;}
      else{ return 0;}
    }
  }
  
  int val = 0;
  
  int judge = r-l-1;
  if( judge == dfs(l+1,r-1) && abs(w[l]-w[r]) <= 1 ){ val = r-l+1;}
  //この範囲は全部取り除ける
  else{
    //全部は取り除けない→左右に分割可能(必ず融合しないため)
    for(int k = l; k < r; k++){
      val = max(val, dfs(l,k)+dfs(k+1,r));
    }
    
  }
    
  return memo[l][r] = val;
}

signed main(void){ 
 while(cin>>n){
   
   if( n == 0 ){ return 0;}
   
   for(int i = 0; i < n; i++){ cin >> w[i];}
   
   for(int i = 0; i < 310; i++){
     for(int j = 0; j < 310; j++){
       memo[i][j] = -1;
     }
   }
   
   cout << dfs(0,n-1) << endl;
   
 }
  
  return 0;
}