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 <stdio.h>
#include <cmath>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
typedef long long int ll;
#define BIG_NUM 2000000000
#define MOD 1000000007
using namespace std;

bool** table;
int** dp;
int N;

int recursive(int left,int right){

	if(left == right)return dp[left][right] = 0;
	if(dp[left][right] != -1)return dp[left][right]; //?????¢???????????´???
	if((right-left+1)%2 == 1){ //??????????\???°?????´???
		return dp[left][right] = max(recursive(left,right-1),recursive(left+1,right));
	}
	if(table[left][(right-left+1)/2] == true)return dp[left][right] = right-left+1; //left???right?????¨????¶?????????´???
	if(right == left+1 && table[left][1] == false)return 0;

	int ans = 0;
	for(int i = left+1; i <= right-2; i++){
		ans = max(ans,recursive(left,i)+recursive(i+1,right));
	}
	return dp[left][right] = ans;
}

int main(){

	int data[301];
	table = new bool*[301]; //table[left][right] = left???????§?????????????2*right??????????????¨?¶??????§??????????????????
	dp = new int*[301]; //?????????????????????????????§?¶????????????°
	for(int i = 0; i <= 300; i++){
		table[i] = new bool[151];
		dp[i] = new int[151];
	}

	bool FLG;

	while(true){
		scanf("%d",&N);
		if(N == 0)break;

		if(N <= 1){
			printf("0\n");
			continue;
		}

		//?¶??????????????????????????????????
		for(int i = 0; i <= 300; i++){
			for(int k = 0; k <= 150; k++)table[i][k] = false;
			for(int k = 0; k <= 300; k++){
				dp[i][k] = -1;
			}
		}

		for(int i = 1; i <= N; i++)scanf("%d",&data[i]);

		//????????????2???????¨?
		for(int i = 1; i <= N-1; i++){
			if(abs(data[i+1]-data[i]) <= 1)table[i][1] = true;

		}

		//?¶??????????????????????????¨????
		for(int length = 4; length <= N; length += 2){ //??????????????????
			for(int left = 1; left+length -1 <= N; left++){ //??????????????????
				FLG = false;

				if(table[left+1][(length-2)/2] == true && abs(data[left] - data[left+length-1]) <= 1)FLG = true;//??????????????£?????????length-2??????????¶????????????????????????¨??????+length-1?????´???????????????????¶?????????´???

				if(!FLG){
					for(int i = 2; i <= length-2; i += 2){
						if(table[left][i/2] == true && table[left+i][(length-i)/2] == true){
							FLG = true;
							break;
						}
					}
				}
				if(FLG){
					table[left][length/2] = true;
				}
			}
		}

		printf("%d\n",recursive(1,N));
	}

	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int dp[300][300];
int w[300];
int rec(int s,int t){
  if(dp[s][t]!=-1)return dp[s][t];
  if(s==t)return 0;
  //if((t-s+1)%2)return max(rec(s,t-1),rec(s+1,t));
  int ans=rec(s+1,t-1);
  if(ans==t-s-1&&abs(w[s]-w[t])<=1)return dp[s][t]=ans+2;
  for(int i=s;i<t;i++){
    ans=max(ans,rec(s,i)+rec(i+1,t));
  }
  return dp[s][t]=ans;
}

int main(){
  int n;
  while(cin>>n,n){
    memset(w,0,sizeof(w));
    for(int i=0;i<n;i++)cin>>w[i];
    memset(dp,-1,sizeof(dp));
    for(int i=1;i<n;i++)
      dp[i-1][i]=(abs(w[i-1]-w[i])<=1?2:0);
    cout<<rec(0,n-1)<<endl;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
 
#define rep(i,n) for(ll i=0;i<n;++i)
#define ALL(a) (a).begin(),(a).end()
 
using namespace std;
using ll  = long long;
using pll = pair<ll, ll>;
 
const ll MOD  = 1e9+7;
const ll LINF = 1LL << 60;
const int INF = 1e9 + 7;

int main(){
    vector<int> ans;
    while(true){
        int n;
        cin >> n;
        if(n == 0)break;

        vector<int> w(n);
        rep(i, n)cin >> w[i];

        ll dp[n+1][n+1] = {};

        //i個の達磨を考える
        for(int i = 2; i <= n; ++i){
            for(int l = 0; l+i-1 < n; ++l){
                int r = i+l-1;

                //真ん中が全部消せるのであればlとrが隣あう
                if(dp[l+1][r-1] == i-2 && abs(w[l] - w[r]) <= 1){
                    dp[l][r] = dp[l+1][r-1] + 2;
                }
                
                for(int c = l; c < r; ++c){
                    dp[l][r] = max(dp[l][r], dp[l][c] + dp[c+1][r]);
                }
             }
        }

        ans.push_back(dp[0][n-1]);
    }

    rep(i, ans.size())cout << ans[i] << endl;
}

C++: //#define __USE_MINGW_ANSI_STDIO 0
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<int, int> PII;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define IN(a, b, x) (a<=x&&x<b)
#define MP make_pair
#define PB push_back
#define MOD 1000000007
#define INF (1LL<<30)
#define LLINF (1LL<<60)
#define PI 3.14159265359
#define EPS 1e-12
//#define int ll

int dx[] = {0, 1, 0, -1}, dy[] = {1, 0, -1, 0};

int w[305], dp[305][305];
signed main(void)
{
	while(true) {
		int n;
		cin >> n;
		if(!n) break;
		REP(i, n) cin >> w[i];
		REP(i, 305) REP(j, 305) dp[i][j] = 0;

		FOR(W, 2, n+1) {
			REP(i, n) {
				int j = i+W-1;
				if(j >= n) continue;
				//cout << i << " " << j << endl;

				if(dp[i+1][j-1] == W-2 && abs(w[i] - w[j]) <= 1) dp[i][j] = W;
				FOR(k, i, j) {
					dp[i][j] = max(dp[i][j], dp[i][k] + dp[k+1][j]);
					//cout << dp[i][j] << endl;
				}
			}
		}
		/*REP(i, n) {
			cout << i << ":";
			REP(j, n) {
				cout << dp[i][j] << " ";
			}
			cout << endl;
		}*/
		cout << dp[0][n-1] << endl;
	}


	return 0;
}