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 <array>
#include <cassert>
#include <cmath>
#include <cstring>
#include <iostream>

int blocks[300];
int8_t strike_range[300][300];
int score_in_range[300][300];

const int8_t STRIKE_RANGE_UNDEF = 0;
const int8_t STRIKE_RANGE_NO = 1;
const int8_t STRIKE_RANGE_YES = 2;

bool can_strike(int x, int y) {
	return std::abs(blocks[x] - blocks[y]) <= 1;
}

int8_t search_range(int start, int end) {
	if (start >= end) return STRIKE_RANGE_UNDEF;
	if ((end - start + 1) % 2 != 0) return STRIKE_RANGE_NO;

	int8_t* p = &strike_range[start][end];
	if (*p != STRIKE_RANGE_UNDEF) return *p;

	if (start + 1 == end)
		return *p = can_strike(start, end) ? STRIKE_RANGE_YES : STRIKE_RANGE_NO;

	for (int i = start + 1; i < end - 1; i++) {
		if (search_range(start, i) == STRIKE_RANGE_YES
			&& search_range(i + 1, end) == STRIKE_RANGE_YES)
		{
			return *p = STRIKE_RANGE_YES;
		}
	}

	auto result = search_range(start + 1, end - 1);
	return *p = result == STRIKE_RANGE_YES && can_strike(start, end)
		? STRIKE_RANGE_YES : STRIKE_RANGE_NO;
}

int main()
{
	while (true) {
		int n;
		std::cin >> n;

		if (n == 0) return 0;

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

		std::memset(strike_range, 0, sizeof(strike_range));

		for (int i = 0; i < n; i++) {
			for (int j = 0; j < n; j++) {
				score_in_range[i][j] = search_range(i, j) == STRIKE_RANGE_YES ? j - i + 1 : 0;
			}
		}

		for (int i = 0; i < n; i++) {
			for (int j = 0; j < i; j++) {
				score_in_range[0][i] = std::max(
					score_in_range[0][i],
					score_in_range[0][j] + score_in_range[j + 1][i]
				);
			}
		}

		std::cout << score_in_range[0][n - 1] << std::endl;
	}
}

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 > 2) {
                    int len2 = len - 2;
                    if ((dp[l][l+1] && dp[l+2][l+2+len2-1]) ||
                        (dp[l][l+len2-1] && dp[l+len2][l+len2+1])) {
                        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<iostream>
#include<vector>
#include<algorithm>
#include<cmath>
using namespace std;
int n, a[1000], dp[1000][1000];
int main() {
	while (true) {
		cin >> n; if (n == 0)break; for (int i = 0; i < 1000; i++) { for (int j = 0; j < 1000; j++)dp[i][j] = 0; }
		for (int i = 0; i < n; i++)cin >> a[i];
		for (int i = 0; i < n - 1; i++) { if (abs(a[i] - a[i + 1]) <= 1) { dp[i][i + 1] = 2; } }
		for (int h = 0; h < 30; h++) {
			for (int i = 2; i <= n; i++) {
				for (int j = 0; j < n; j++) {
					int a1 = j, a2 = i + j - 1; if (a2 >= n)continue;
					if (dp[a1][a2] == 0)continue;
					if (a1 > 0 && a2 < n - 1 && abs(a[a1 - 1] - a[a2 + 1]) <= 1)dp[a1 - 1][a2 + 1] = i + 2;
				}
			}
			for (int i = 1; i <= n; i++) {
				for (int j = 0; j <= n; j++) {
					int a1 = j, a3 = i + j; if (a3 >= n)continue;
					for (int k = a1 + 1; k <= a3; k++) {
						int a2 = k;
						dp[a1][a3] = max(dp[a1][a3], dp[a1][a2 - 1] + dp[a2][a3]);
					}
				}
			}
		}
		cout << dp[0][n - 1] << endl;
	}
	return 0;
}