Problem
A message containing letters from A-Z
is being encoded to numbers using the following mapping:
'A' -> 1
'B' -> 2
...
'Z' -> 26
Given a non-empty string containing only digits, determine the total number of ways to decode it.
Example 1:
Input: "12"
Output: 2
Explanation: It could be decoded as "AB" (1 2) or "L" (12).
Example 2:
Input: "226"
Output: 3
Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
Solution
/**
* @param {string} s
* @return {number}
*/
var numDecodings = function(s) {
var len = s.length;
var tmp = 0;
var tmp1 = 1;
var tmp2 = 0;
var num1 = 0;
var num2 = 0;
if (s === '' || s[0] === '0') return 0;
for (var i = 1; i <= len; i++) {
num1 = Number(s.substr(i - 1, 1));
num2 = Number(s.substr(i - 2, 2));
if (num1 !== 0) tmp += tmp1;
if (10 <= num2 && num2 <= 26) tmp += tmp2;
tmp2 = tmp1;
tmp1 = tmp;
tmp = 0;
}
return tmp1;
};
Explain:
每个点可由前面一个点 decode 1 一个数字到达或前面两个点 decode 2 个数字到达。
Complexity:
- Time complexity : O(n).
- Space complexity : O(1).