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;

int n;
vector<int> w;
vector<vector<int>> dp;

int rec(int l, int r)
{
  if (~dp.at(l).at(r))
  {
    return dp.at(l).at(r);
  }
  if (l == r)
  {
    return dp.at(l).at(r) = 0;
  }
  if (l + 1 == r)
  {
    if (abs(w.at(l) - w.at(r)) <= 1)
    {
      return dp.at(l).at(r) = 2;
    }
    return dp.at(l).at(r) = 0;
  }
  int tmp = 0;
  if (abs(w.at(l) - w.at(r)) <= 1 && rec(l + 1, r - 1) == r - l - 1)
  {
    tmp = r - l + 1;
  }
  for (int m = l; m < r; m++)
  {
    tmp = max(tmp, rec(l, m) + rec(m + 1, r));
  }
  return dp.at(l).at(r) = tmp;
}

int main()
{
  vector<int> ans;
  while (1)
  {
    cin >> n;
    if (!n)
    {
      break;
    }
    w.resize(n);
    for (int i = 0; i < n; i++)
    {
      cin >> w.at(i);
    }
    dp.resize(n, vector<int>(n));
    dp.assign(n, vector<int>(n, -1));
    ans.push_back(rec(0, n - 1));
  }
  for (int i = 0; i < ans.size(); i++)
  {
    cout << ans.at(i) << endl;
  }
}

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

int calc(vector<int> &data, vector<vector<int>> &dp, int l, int r) {
    int ret = 0;
    if(dp[l][r] != -1) return dp[l][r];
    if(abs(r-l) <= 1) return 0;

    if(abs(data[l]-data[r-1]) < 2 && calc(data, dp, l+1, r-1) == r-l-2) ret = r - l;

    for(int mid=l+1; mid<r; mid++) {
        ret = max(ret, calc(data, dp, l, mid)  + calc(data, dp, mid, r));
    }

    dp[l][r] = ret;
    //cout<<"dp["<<l<<"]["<<r<<"] = "<<dp[l][r]<<endl;
    return ret;

}

int solve() {
    int n;
    cin >> n;
    if(n==0) return 0;

    vector<vector<int>> dp;
    dp.resize(n+1);
    for(auto &item: dp) {
        item.resize(n+1, -1);
    }
    vector<int> data;
    data.resize(n);

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


    cout << calc(data, dp, 0, n) << endl;

    return 1;
}

int main() {
    while(solve());
}

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

void init() {}

int N;
int W[303];
int dp[303][303];
bool done[303][303];
int rec(int l, int r) {
    if (r - l < 2) return 0;
    if (r == l + 2) {
        if (abs(W[l] - W[l + 1]) <= 1)
            return dp[l][r] = 2;
        else
            return dp[l][r] = 0;
    }
    if (done[l][r]) return dp[l][r];
    done[l][r] = true;
    if (abs(W[l] - W[r - 1]) <= 1) {
        if (rec(l + 1, r - 1) == r - l - 2) {
            return dp[l][r] = r - l;
        }
    }
    int res = 0;
    FOR(i, l + 1, r) { res = max(res, rec(l, i) + rec(i, r)); }
    return dp[l][r] = res;
}
bool solve() {
    cin >> N;
    if (N == 0) return false;
    REP(i, N) cin >> W[i];
    REP(i, N + 1) REP(j, N + 1) {
        done[i][j] = false;
        dp[i][j] = 0;
    }
    cout << rec(0, N) << endl;
    return true;
}
signed main() {
    init();
    while (solve())
        ;
}