求 dp[0][n-1]
算出所有的 dp[L][R] , 0<=L<n , L<=R<n
我由 R-L+1 = 1 開始加寬[L,R]間距，求至 R-L+1 = n

dp[L][R] = 1 , for L=R
dp[L][R] = dp[L][R-1], for c[L]=c[R] 或 c[R-1]=c[R]
dp[L][R] = min(dp[L][x]+dp[x+1][R]) , for L<=x<R