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 <iostream>
#include <algorithm>
using namespace std;

int n, w[300], dp[300][300], dp2[300][300];

int main() {

	while (true) {

		cin >> n;
		if (n == 0)break;

		for (int i = 0; i < n; i++) {
			cin >> w[i];
		}

		for (int i = 0; i < n - 1; i++) {
			if (abs(w[i] - w[i + 1]) <= 1) {
				dp[i][i + 1] = 2;
			}
		}

		for (int i = 3; i < n; i += 2) {//区間の大きさ
			for (int j = 0; j < n - i; j++) {
				//両端の計算
				if (abs(w[j] - w[j + i]) <= 1 && dp[j + 1][j + i - 1] > 0) {
					dp[j][j + i] = dp[j + 1][j + i - 1] + 2;
				}
				for (int k = j + 1; k < j + i; k += 2) {
					//jからkを中間とした長i+1区間を取り出せるかどうか

					if (dp[j][k] > 0 && dp[k + 1][j + i] > 0) {
						dp[j][j + i] = max(dp[j][j + i], dp[j][k] + dp[k + 1][j + i]);
					}
				}
			}
		}

		//区間での取り出せる個数の最大を計算
		for (int i = 2; i < n; i++) {
			for (int j = 0; j < n - i; j++) {
				//dpの方の取り出せる数を先に入れる
				dp2[j][j + i] = dp[j][j + i];

				for (int k = j + 1; k < j + i; k++) {
					//jからkを中間とした長i+1区間の取り出せる個数の最大

					//ここが間違っている
					dp2[j][j + i] = max(dp2[j][j + i], dp2[j][k] + dp2[k + 1][j + i]);
					dp2[j][j + i] = max(dp2[j][j + i], dp2[j][k] + dp[k + 1][j + i]);
					dp2[j][j + i] = max(dp2[j][j + i], dp[j][k] + dp2[k + 1][j + i]);
					dp2[j][j + i] = max(dp2[j][j + i], dp[j][k] + dp[k + 1][j + i]);
				}
			}
		}

		cout << dp2[0][n - 1] << endl;

		/*for (int i = 0; i < n; i++) {
			for (int j = 0; j < n; j++) {
				cout << dp[i][j] << " ";
			}
			cout << endl;
		}*/

		for (int i = 0; i < n; i++) {
			for (int j = 0; j < n; j++) {
				dp[i][j] = dp2[i][j] = 0;
			}
		}
	}
}

C++: #include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
#define all(x) x.begin(),x.end()
#define dbg(x) cout<<#x<<":"<<x<<endl
#define int long long
#define MOD 1000000007
typedef pair<int,int> P;
typedef pair<int,P> PP;
int n,w[333];
int dp[333][333];

int rec(int l,int r){
  if(dp[l][r]!=-1)return dp[l][r];
  dp[l][r]=0;
  for(int i=l+1;i<r;i++){
    int res=0;
    int suml=rec(l+1,i);
    int sumr=rec(i+1,r);
    if(suml==i-l-1&&abs(w[l]-w[i])<2)res+=2;
    dp[l][r]=max(dp[l][r],res+suml+sumr);
  }
  return dp[l][r];
}

signed main(){
  while(1){
  cin>>n;
  if(n==0)break;
  for(int i=0;i<n;i++)cin>>w[i];
  fill_n(*dp,333*333,-1);
  for(int i=0;i<n;i++){
    dp[i][i]=0;
    dp[i][i+1]=0;
  }
  cout<<rec(0,n)<<endl;
  }
  return 0;
}

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

#define DUMP(x) cerr << #x << "=" << x << endl
#define DUMP2(x, y) cerr<<"("<<#x<<", "<<#y<<") = ("<<x<<", "<<y<<")"<< endl
#define BINARY(x) static_cast<bitset<16> >(x)

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define REP(i,m,n) for (int i=m;i<(int)(n);i++)

#define in_range(x, y, w, h) (0<=(int)(x) && (int)(x)<(int)(w) && 0<=(int)(y) && (int)(y)<(int)(h))
#define ALL(a) (a).begin(),(a).end()

typedef long long ll;
const int INF   = 1e9;
const ll  INFLL = 1e18;
typedef pair<int, int> PII;
int dx[4]={0, -1, 1, 0}, dy[4]={-1, 0, 0, 1};

// #define DEBUG

const int MAX_N = 300;
int main()
{
    ios::sync_with_stdio(false);

    int N;
    while (cin >> N, N) {
        vector<int> W(N);
        rep(i, N) cin >> W[i];

        bool dp[MAX_N][MAX_N] = {};
        rep(i, MAX_N) {
            dp[i][i] = true;
        }

        for (int len=2; len<=N; len+=2) {
            for (int l=0; l<=N-len; l++) {
                {
                    int inner_len = len - 2;
                    if (!(inner_len > 0 && !dp[l + 1][l + inner_len]) && abs(W[l] - W[l+ (len - 1)]) <= 1) {
                        dp[l][l + (len - 1)] = true;
                    }
                }
                if (len >= 4) {
                    bool ok = true;
                    for (int l2=l; l2<l+len; l2+=2) {
                        if (!dp[l2][l2+1]) {
                            ok = false;
                            break;
                        }
                    }
                    if (ok) {
                        dp[l][l + (len - 1)] = true;
                    }
                }
            }
        }

        int dp2[MAX_N + 1] = {};
        rep(r, N) {
            if (r > 0) dp2[r] = dp2[r-1];
            for (int len=2; len<=r+1; len+=2) {
                int l = r - len + 1;
#ifdef DEBUG
                // DUMP(len);
                // DUMP2(l, r);
#endif
                if (dp[l][r]) {
                    if (l-1 >= 0) {
                        dp2[r] = max(dp2[r], dp2[l-1] + len);
                    } else {
                        dp2[r] = max(dp2[r], len);
                    }
                }
            } 
        }

#ifdef DEBUG
        rep(l, N) {
            rep(r, N) cerr << dp[l][r] << " ";
            cerr << endl;
        }
        cerr << "-----------------------------" << endl;
        rep(i, N) cerr << dp2[i] << " ";
        cerr << endl;
#endif

        cout << dp2[N-1] << endl;
    }
}

C++: #include <bits/stdc++.h>

using namespace std;

#define rep(i, n) for (int i = 0; i < (n); ++i)

int main() {
  while (1) {
    int n;
    cin >> n;
    if (n == 0)
      break;
    int w[n];
    rep(i, n) cin >> w[i];

    vector<vector<int> > dp(n + 1, vector<int>(n + 1));
    for (int W = 2; W <= n; ++W) {
      rep(l, n) {
        int r = l + W;
        if (r > n)
          continue;
        if (dp[l + 1][r - 1] == W - 2 && abs(w[l] - w[r - 1]) <= 1) {
          dp[l][r] = W;
        }

        for (int m = l; m <= r; ++m) {
          dp[l][r] = max(dp[l][r], dp[l][m] + dp[m][r]);
        }
      }
    }

    cout << dp[0][n] << endl;
  }
  return 0;
}