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[301];
	}

	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 <iostream>
#include <vector>
#include <algorithm>
#include <cmath>

#define show(x) cerr << #x << " = " << x << endl
using namespace std;

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "sz:" << v.size() << "\n[";
    for (const auto& p : v) {
        os << p << ",";
    }
    os << "]\n";
    return os;
}

template <typename S, typename T>
ostream& operator<<(ostream& os, const pair<S, T>& p)
{
    os << "(" << p.first << "," << p.second
       << ")";
    return os;
}


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

        vector<vector<bool>> ok(n, vector<bool>(n, false));  // [i,j]?????¨????????¨?????????
        for (int d = 1; d < n; d += 2) {
            for (int i = 0; i + d < n; i++) {
                if (d == 1) {
                    if (abs(w[i] - w[i + d]) <= 1) {
                        ok[i][i + d] = true;
                    }
                } else {
                    if (ok[i + 1][i + d - 1] and abs(w[i] - w[i + d]) <= 1) {
                        ok[i][i + d] = true;
                    } else if (ok[i + 2][i + d] and abs(w[i] - w[i + 1]) <= 1) {
                        ok[i][i + d] = true;
                    }
                }
            }
        }
        vector<pair<int, int>> interval;
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                if (ok[i][j]) {
                    interval.push_back(make_pair(i + 1, j + 1));
                }
            }
        }
        const int size = interval.size();
        vector<vector<int>> dp(size + 1, vector<int>(n + 1, 0));
        for (int i = 0; i < size; i++) {
            for (int j = 0; j <= n; j++) {
                dp[i + 1][j] = max(dp[i + 1][j], dp[i][j]);
                if (j < interval[i].first) {
                    dp[i + 1][interval[i].second] = max(dp[i + 1][interval[i].second], dp[i][j] + (interval[i].second - interval[i].first + 1));
                }
            }
        }

        int maxi = 0;
        for (int i = 0; i <= n; i++) {
            maxi = max(maxi, dp[size][i]);
        }
        cout << maxi << endl;
    }

    return 0;
}