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0091-decode-ways.cpp
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0091-decode-ways.cpp
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#include<string>
using namespace std;
//Solution 1: Recursion
//class Solution {
//public:
// int recur(string &s, int idx, int n, vector<int> &memo) {
// if (idx >= n) return 1;
// if (s[idx] == '0') return 0;
// if (idx == n-1) return 1;
// if (memo[idx] > -1) return memo[idx];
//
// int sng = recur(s, idx+1, n, memo);
// int one = 0;
// if (s[idx] == '1')
// one = (recur(s, idx+2,n, memo));
// int two = 0;
// if(s[idx] == '2' && idx < n-1 && s[idx+1] <= '6')
// two = (recur(s, idx+2, n, memo));
// return memo[idx] = sng+one+two;
// }
// int numDecodings(string s) {
// vector<int> memo (s.size(), -1);
// return recur(s, 0, s.size(), memo);
// }
//};
//Solution 2: Tabulation
//class Solution {
//public:
// int numDecodings(string s) {
// int n = s.size();
// vector<int> memo (n, 0);
// memo[n-1] = s[n-1] != '0';
// if (n>1) memo[n-2] = s[n-2]=='0'?0:((s[n-2] == '1' ? 1:0) + (s[n-2] == '2' && s[n-1] < '7' ? 1:0) + memo[n-1]);
// for (int i=n-3;i>=0;i--) {
// if (s[i] == '0') {
// memo[i] = 0;
// continue;
// }
// int sng = memo[i+1];
// int one = 0;
// if (s[i] == '1')
// one = memo[i+2];
// int two = 0;
// if(s[i] == '2' && i < n-1 && s[i+1] <= '6')
// two = (memo[i+2]);
// memo[i] = sng+one+two;
// }
// return memo[0];
// }
//};
//Solution 3: Optimised tabulation
class Solution {
public:
int numDecodings(string s) {
int n = s.size();
int plus_two = s[n - 1] != '0';
if (n == 1)
return plus_two;
int plus_one =
s[n - 2] == '0'
? 0
: ((s[n - 2] == '1' ? 1 : 0) +
(s[n - 2] == '2' && s[n - 1] < '7' ? 1 : 0) + plus_two);
int curr;
for (int i = n - 3; i >= 0; i--) {
if (s[i] == '0') curr = 0;
else {
curr = plus_one;
if (s[i] == '1')
curr += plus_two;
if (s[i] == '2' && i < n - 1 && s[i + 1] <= '6')
curr += (plus_two);
}
plus_two = plus_one;
plus_one = curr;
}
return plus_one;
}
};