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>
#define INF 2000000000
#define int long long int
#define MOD 1000000007
#define pb push_back
#define cin1(n) (cin >> n)
#define cin2(a,b) (cin >> a >> b)
#define cin3(a,b,c) (cin >> a >> b >> c)
#define cout(n) (cout << n << endl)
#define rep(i,a,b) for(int i=(a);i<(b);++i)
#define fr first 
#define sc second 
#define EPS (1e-10)

using namespace std;

typedef vector<int> vi;
typedef vector< vector<int> > vvi;
typedef vector<pair<int,int>> vpii;
typedef vector<string> vs;
typedef vector<vector<string>> vss;
typedef pair<int,int> pii;

int dx[] = {1,-1,0,0};
int dy[] = {0,0,1,-1};
int n,m;
int a,b;
int p,q;


signed main() {
	
	while(1) {
		cin1(n);
		if (n == 0) break;

		vvi dp(n,vi(n,0)); //dp[i][j] i??????j??????????????§?????°
		vi daruma(n,0);
		rep(i,0,n) {
			cin1(daruma[i]);
		}

		rep(i,1,n) {
			if (abs(daruma[i-1] - daruma[i]) <= 1) {
				dp[i-1][i] = 2;
			}
		}

		for (int i = 2; i <= n; i++) { //??????
			for (int l = 0; l < n - i + 1; l++) { //??????
				int r = l + i - 1;
				if (dp[l+1][r-1] == i - 2 and abs(daruma[l]-daruma[r]) <= 1) dp[l][r] = i; 
				for (int k = l; k < r; k++) {
					dp[l][r] = max(dp[l][r],dp[l][k] + dp[k+1][r]);	
				}
				
			}
		}

		cout(dp[0][n-1]);
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
using namespace std;

int N, w[1000];
bool dp[1000][1000];

bool rec(int left, int right) {
    if(dp[left][right]) return true;
    if(left >= right) return false;

    bool ret = false;
    if(!(abs(left - right) % 2)) {
        rec(left + 1, right), rec(left, right - 1);
    } else {
        for(int i = left + 1; i < right; i += 2) {
            ret |= rec(left, i) & rec(i + 1, right);
        }
        ret |= rec(left + 1, right - 1) & (abs(w[left] - w[right]) <= 1);
    }
    return dp[left][right] = ret;
}

int maxi[1001];
int calc(int idx) {

    if(maxi[idx]) return maxi[idx];
    if(idx > N) return 0;

    int ret = 0;
    ret = calc(idx + 1);
    for(int i = idx + 1; i < N; i += 2) {
        if(dp[idx][i]) {
            ret = max(calc(i + 1) + (i - idx + 1), ret);
        }
    }
    return maxi[idx] = ret;
}

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

        fill_n(maxi, 1001, 0);
        fill_n(*dp, 1000*1000, false);
        for(int i = 0; i < N - 1; i++) {
            if(abs(w[i] - w[i + 1]) <= 1) {
                dp[i][i + 1] = true;
            }
        }
        rec(0, N - 1);
        cout << calc(0) << endl;
    }
}