public int numDecodings(String s) {

int length = s.length();

length = s.length();

if (length == 0) {

return 0;

}

if (s.charAt(0) == '0') {

return 0;

}

int[] dp = new int[length + 1];

dp[0] = 1;

dp[1] = 1;

boolean isZero = false;

for (int i = 2; i <= length; i++) {

int n = Integer.valueOf(s.substring(i-2, i));

if (s.charAt(i - 1) == '0' && (n > 26 || n == 0)) {

return 0;

} else if (s.charAt(i - 1) == '0') {

isZero = true;

dp[i] = dp[i - 2];

} else if (isZero) {

isZero = false;

dp[i] = dp[i - 1];

} else {

isZero = false;

dp[i] = dp[i - 1] + ((n > 0 && n <= 26) ? dp[i - 2] : 0);

}

}

return dp[length];

}

/**

public int numDecodings2(String s) {

int n = s.length();

if (n == 0) return 0;

int[] memo = new int[n+1];

memo[n] = 1;

memo[n-1] = s.charAt(n-1) != '0' ? 1 : 0;

for (int i = n - 2; i >= 0; i--)

if (s.charAt(i) == '0') continue;

else memo[i] = (Integer.parseInt(s.substring(i,i+2))<=26) ? memo[i+1]+memo[i+2] : memo[i+1];

return memo[0];

}

/**

public int numDecodings3(String s) {

if (s == null || s.length() == 0) {

return 0;

}

int[] ways = new int[s.length() + 1];

ways[0] = 1;

ways[1] = s.charAt(0) == '0' ? 0 : 1;

for (int i = 2; i <= s.length(); i++) {

int one = s.charAt(i - 1) - '0';

int two = (s.charAt(i - 2) - '0') * 10 + one;

if (one != 0) {

ways[i] += ways[i - 1];

}

if (two >= 10 && two <= 26) {

ways[i] += ways[i - 2];

}

}

return ways[s.length()];

}

public int numDecodings4(String s) {

if (s == null || s.length() == 0) return 0;

if (s.charAt(0) == '0') return 0;

int[] dp = new int[s.length()+1];

dp[0] = 1;

dp[1] = 1;

for (int i=2; i<=s.length(); i++) {

int now = s.charAt(i-1) - '0';

int prenow = (s.charAt(i-2) - '0')*10 + now;

if (now == 0 && prenow != 10 && prenow != 20) return 0;

int a = now == 0 ? 0 : dp[i-1];

int b = (prenow >= 10 && prenow <= 26) ? dp[i-2] : 0;

dp[i] = a + b;

}

return dp[s.length()];

}

/**

public int numDecodings5(String s) {

int n1 =1, n2=1, n3=0;

if(s.length() == 0 || s.charAt(0) == '0') return 0;

for (int i=2; i<=s.length(); i++) {

n3=0;

if(s.charAt(i-1)!='0') n3=n2;

int num = Integer.parseInt(s.substring(i-2,i));

if(num>=10 && num<=26) n3+=n1;

n1=n2;

n2=n3;

}

return n2;

}