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 int long long
#define MKP make_pair
typedef pair<int, int> pii;

constexpr int MAX_N = 305;
constexpr int INF = 1LL << 60;

int n, mem[MAX_N][MAX_N], w[MAX_N];

int solve(int l, int r) {
    if (r - l <= 1) {
        return 0;
    }
    if (mem[l][r] != -1) {
        return mem[l][r];
    }

    int res = max(solve(l + 1, r), solve(l, r - 1));

    if (abs(w[l] - w[l + 1]) <= 1) {
        res = max(res, solve(l + 2, r) + 2);
    }
    if (abs(w[r - 1] - w[r - 2]) <= 1) {
        res = max(res, solve(l, r - 2) + 2);
    }
    if (abs(w[l] - w[r - 1]) <= 1) {
        int tmp = solve(l + 1, r - 1);
        if (tmp == r - 1 - (l + 1)) {
            res = max(res, tmp + 2);
        }
    }

    return mem[l][r] = res;
}

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    while(1) {
        cin >> n;
        if (n == 0) {
            return 0;
        }
        memset(mem[0], -1, sizeof(mem));

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

        cout << solve(0, n) << endl;
    }
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <set>
#include <map>
#include <string.h>
#include <cmath>
#include <functional>

using namespace std;
typedef long long i64;
typedef long double ld;
typedef pair<i64,i64> P;

int n;

//[l,r)
		function<int(int,int)> rec = [&](int l,int r){
			if(dp[l][r] != -1) return dp[l][r];

			if(abs(l - r) <= 1) return 0;

			int res = 0;
			//l [able] r ?
			if(abs(w[l] - w[r - 1]) <= 1 && rec(l + 1,r - 1) == r - l - 2)
			{
				res = r - l;
			}

			//[l any] [any r]
			for(int mid = l + 1;mid <= r - 1;mid++)
			{
				res = max(res , rec(l,mid) + rec(mid,r));
			}
			return dp[l][r] = res;
		};

int main()
{
	while(cin >> n , n)
	{
		vector<int> w(n,0);
		for(int i = 0;i < n;i++) cin >> w[i];

		vector<vector<int>> dp(n + 1,vector<int>(n + 1,-1));

		

		cout << rec(0,n) << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
#define MOD 998244353
template<class T>inline bool chmax(T& a,T b){if (a<b){a=b;return 1;}return 0;}
template<class T>inline bool chmin(T& a,T b){if (a>b){a=b;return 1;}return 0;}
const long long INF = 1LL << 60;
bool pairCompare(const pair<double,ll>& firstElof, const pair<double,ll>& secondElof){
    return firstElof.first < secondElof.first;
}
bool pairCompareSecond(const pair<double,ll>& firstElof, const pair<double,ll>& secondElof){
    return firstElof.second < secondElof.second;
}
//四方向への移動ベクトル
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

struct edge{//グラフに使うヤツ
    ll from,to,cost;
};

typedef vector<vector<edge> > G;
ll gcd(ll a,ll b){
    if (a%b==0)return(b);
    else return(gcd(b,a%b));
}

int main() {
    while(true){
        ll n;
        cin >> n;
        if (n==0)break;
        ll w[n];
        for (ll i=0;i<n;i++)cin >> w[i];
        ll dp[n+1][n+1];
        memset(dp,0,sizeof(dp));
        for (ll i=2;i<=n;i++){
            for (ll l=0;l<=n;l++){
                ll r=i+l;
                if (i+l>n){
                    continue;
                }
                if (dp[l+1][r-1]==i-2 and abs(w[l]-w[r-1])<=1)dp[l][r]=i;
                for (ll mid=l;mid<=r;mid++){
                    dp[l][r]=max(dp[l][r],dp[l][mid]+dp[mid][r]);
                }
            }
        }
        cout << dp[0][n] << 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){
            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>
#define r(i,n) for(int i=0;i<n;i++)
using namespace std;
int a[301],n,dp[301][301];
main(){
  while(cin>>n,n){
    int ans=0;
    r(i,n)cin>>a[i];
    memset(dp,0,sizeof(dp));
    for(int i=1;i<n;i+=2)
      for(int j=0;j<n-i;j++){
        if(abs(a[j]-a[j+i])<2&&dp[j+1][j+i-1]==i-1)dp[j][j+i]=max(dp[j][j+i],i+1);
        else for(int k=1;k<i;k+=2)dp[j][j+i]=max(dp[j][j+i],dp[j][j+k]+dp[j+k+1][j+i]);
      }
    r(i,n)r(j,n)ans=max(ans,dp[i][j]);
    cout<<ans<<endl;
  }
}