A message containing letters from
A-Zis being encoded to numbers using the following mapping:‘A‘ -> 1 ‘B‘ -> 2 ... ‘Z‘ -> 26Given an encoded message containing digits, determine the total number of ways to decode it.
For example,
Given encoded message"12", it could be decoded as"AB"(1 2) or"L"(12).The number of ways decoding
"12"is 2.
一维动态规划，从后往前。sum[i]只与sum[i + 1] 和sum[i + 2]相关。PS：可以降低空间复杂度为O(1)。public int numDecodings(String s) { if (s == null || s.length() == 0) { return 0; } int len = s.length(); int[] num = new int[len + 1]; num[len] = 1; char last = s.charAt(len - 1); if (last == '0') { num[len - 1] = 0; } else { num[len - 1] = 1; } for (int i = len - 2; i >=0; i--) { char cur = s.charAt(i); if (cur == '0') { num[i] = 0; } else { String str = s.substring(i, i + 2); int count = Integer.parseInt(str); if (count <= 26) { num[i] = num[i + 1] + num[i + 2]; } else { num[i] = num[i + 1]; } } } return num[0]; }
原文：http://blog.csdn.net/u010378705/article/details/28895911