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;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<VVI> VVVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
typedef long long LL;
typedef priority_queue<int> PQ_DESC;
typedef priority_queue<int, vector<int>, greater<int>> PQ_ASC;
typedef priority_queue<PII> PQ_DESC_PII;
typedef priority_queue<PII, vector<PII>, greater<PII>> PQ_ASC_PII;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef vector<VVLL> VVVLL;

#define SORT_ASC(c) sort((c).begin(), (c).end())
#define SORT_DESC(c) sort((c).begin(), (c).end(), greater<typeof((c).begin())>())
 
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define FORL(i,a,b) for(LL i=(a);i<(b);++i)
#define REPL(i,n)  FORL(i,0,n)
#define SIZE(a) int((a).size())
#define ALL(a) (a).begin(),(a).end()
 
const double EPS = 1e-10;
const double PI  = acos(-1.0);

 //debug func
template<typename T>
void vprint(vector<T> v) {
    for(auto x : v) {
        cerr << x << " ";
    }
    cerr << endl;
}
 
template<typename T>
void vvprint(vector<vector<T>> vv) {
    REP(i, SIZE(vv)) {
        REP(j, SIZE(vv[i])) {
            cerr << vv[i][j] << " ";
        }
        cerr << endl;
    }
}

template <typename Iterator>
inline bool next_combination(const Iterator first, Iterator k, const Iterator last)
{
   /* Credits: Thomas Draper */
   if ((first == last) || (first == k) || (last == k))
      return false;
   Iterator itr1 = first;
   Iterator itr2 = last;
   ++itr1;
   if (last == itr1)
      return false;
   itr1 = last;
   --itr1;
   itr1 = k;
   --itr2;
   while (first != itr1)
   {
      if (*--itr1 < *itr2)
      {
         Iterator j = k;
         while (!(*itr1 < *j)) ++j;
         iter_swap(itr1,j);
         ++itr1;
         ++j;
         itr2 = k;
         rotate(itr1,j,last);
         while (last != j)
         {
            ++j;
            ++itr2;
         }
         rotate(k,itr2,last);
         return true;
      }
   }
   rotate(first,k,last);
   return false;
}

inline double get_time_sec(void){
    return static_cast<double>(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count())/1000000000;
}

template<typename T>
T gcd(T a, T b) {
    if(a > b) swap(a, b);
    if(a == 0) return b;
    else return gcd(b%a, a);
}

template<typename T>
T lcm(T a, T b) {
    return (a / gcd(a, b)) * b;
}

template<typename T>
map<T, T> prime_list(T n) {
    map<T, T> ret;
    for(T i = 2; i*i <= n; i++) {
        if(n % i == 0) {
            ret[i] = 0;
            while(n % i == 0) {
                n /= i;
                ret[i]++;
            }
        }
    }
    if(n != 1) ret[n]++;
    return ret;
}

#define MOD 1000000007

LL mypow(LL a, LL n) {
    if(n == 1) return a % MOD;
    if(n % 2 == 1) return (a * mypow(a, n-1)) % MOD;
    LL t = mypow(a, n/2);
    return (t * t) % MOD; 
}

LL mycomb(LL n, LL k) {
    if(n == 0 || n == k) return 1;
    LL x = 1, y = 1;
    REP(i, k) {
        x = x * (n-i) % MOD;
        y = y * (i+1) % MOD;
    }
    return x * mypow(y, MOD - 2) % MOD;
}

int main(void) {
    while(1) {
        int n;
        cin >> n;
        if(n == 0) return 0;
        VI w(n);
        REP(i, n) cin >> w[i];
        VVI dp(n, VI(n, 0));
        FOR(i, 1, n) {
            REP(l, n) {
                int r = l + i;
                if(r >= n) continue;
                if(i == 1) {
                    if(abs(w[l] - w[r]) <= 1) dp[l][r] = 2;
                } else {
                    if(dp[l+1][r-1] == i - 1 && abs(w[l] - w[r]) <= 1) {
                        dp[l][r] = i+1;
                    } else {
                        FOR(mid, l, r) {
                            dp[l][r] = max(dp[l][r], dp[l][mid] + dp[mid+1][r]);
                        }
                    }
                }

            }
        }
        // vvprint(dp);
        cout << dp[0][n-1] << endl;
    }
}

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

int main() {
  int n;
  while(cin >> n && n) {
    int a[n];
    for(int i=0; i<n; i++) cin >> a[i];
    int dp[n][n];
    memset(dp,0,sizeof(dp));
    for(int i=0; i<n-1; i++) {
      if(abs(a[i]-a[i+1])<=1) dp[i][i+1]=1;
    }
    for(int i=0; i<n-1; i+=2) {
      for(int l=0; l<n; l++) {
        int r=l+i+1;
        if(r>=n) break;
        if(dp[l+1][r-1]&&abs(a[l]-a[r])<=1) dp[l][r]=1;
      }
      for(int l=0; l<n; l++) {
        for(int r=l+1; r<n; r++) {
          if(dp[l][r]) {
            int x=l-i-2;
            if(x>=0) {
              if(dp[x][l-1]) dp[x][r]=1;
            }
            x=r+i+2;
            if(x<n) {
              if(dp[r+1][x]) dp[l][x]=1;
            }
          }
        }
      }
    }
    int d[n+1];
    memset(d,-1,sizeof(d));
    d[0]=0;
    int ans=0;
    for(int i=0; i<n; i++) {
      d[i+1]=max(d[i+1],d[i]);
      if(d[i]==-1) continue;
      for(int j=i+1; j<n; j++) {
        if(dp[i][j]) d[j+1]=max(d[j+1],d[i]+j-i+1);
      }
    }
    for(int i=0; i<=n; i++) ans=max(ans,d[i]);
    cout << ans << endl;
  }
  return 0;
}

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

int main(){
  int n,w[300],maxx[301][301],dp[301][301];
  cin>>n;
  while(n!=0){
    for(int i=0;i<n;i++)
      for(int j=0;j<n;j++){
        dp[i][j]=0;
        maxx[i][j]=0;
      }
    for(int i=0;i<n;i++)
      cin>>w[i];
    for(int i=0;i<n-1;i++)
      if(w[i]-w[i+1]>=-1&&w[i]-w[i+1]<=1)
        dp[i][i+1]=1;
    for(int k=2;k<=n;k++)
      for(int i=0;i+k<n;i++){
        if(dp[i+1][i+k-1]==1&&w[i]-w[i+k]>=-1&&w[i]-w[i+k]<=1)
          dp[i][i+k]=1;
        else
          for(int j=i+1;j<i+k-1;j++)
            if(dp[i][j]==1&&dp[j+1][i+k]==1){
              dp[i][i+k]=1;
              break;
            }
        
      }
    
    for(int k=1;k<n;k++)
      for(int i=0;i+k<n;i++){
        if(dp[i][i+k]==1)
          maxx[i][i+k]=k+1;
        else
          for(int j=i;j<i+k;j++)
            if(maxx[i][i+k]<maxx[i][j]+maxx[j+1][i+k])
              maxx[i][i+k]=maxx[i][j]+maxx[j+1][i+k];
      }
    cout << maxx[0][n-1]<<endl;
    cin>>n;
  }
}