# C++ 59ms AC O(1) space solution, feel the power of if-else!

• ``````class Solution {
public:
int numDecodings(string s) {
if(s.empty() || s.front() == '0') return 0;
int mod = pow(10, 9) + 7;
long p1 = (s[0] == '*') ? 9 : 1, p2 = 1, t;
for(int i = 1; i < s.size(); i++){
if(s[i] == '0') p1 = 0;

if(s[i - 1] != '*' && s[i] != '*'){
if(s[i - 1] == '1' || s[i - 1] == '2' && s[i] < '7'){
p1 = p1 + p2;
p2 = p1 - p2;
}
else p2 = p1;
}
else{
if(s[i - 1] == '*' && s[i] == '*'){
t = p1;
p1 = p1 * 9 + p2 * (9 + 6);
p2 = t;
}
else if(s[i] == '*'){
if(s[i - 1] == '1'){
t = p1;
p1 = p1 * 9 + p2 * 9;
p2 = t;
}
else if(s[i - 1] == '2'){
t = p1;
p1 = p1 * 9 + p2 * 6;
p2 = t;
}else{
t = p1;
p1 = p1 * 9;
p2 = t;
}
}
else{
if(s[i] == '0'){
t = p1;
p1 = p2 * 2;
p2 = t;
}
else if(s[i] < '7'){
t = p1;
p1 = p1 + p2 * 2;
p2 = t;
}
else{
p1 = p1 + p2;
p2 = p1 - p2;
}
}
}
p1 = p1 % mod;
}
return p1;
}
};
``````

• @zefengsong I like the way you phrase it: "feel the power of if-else"!

• @sunfruit Hahaha :p

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