题目地址：https://leetcode.com/problems/distinct-subsequences-ii/description/
题目描述
Given a string S, count the number of distinct, non-empty subsequences of S .
Since the result may be large, return the answer modulo 10^9 + 7.
Example 1:
Input: "abc"
Output: 7
Explanation: The 7 distinct subsequences are "a", "b", "c", "ab", "ac", "bc", and "abc".
Example 2:
Input: "aba"
Output: 6
Explanation: The 6 distinct subsequences are "a", "b", "ab", "ba", "aa" and "aba".
Example 3:
Input: "aaa"
Output: 3
Explanation: The 3 distinct subsequences are "a", "aa" and "aaa".
Note:
1、 Scontainsonlylowercaseletters.；
2、 1<=S.length<=2000；
题目大意
计算一个字符串中，有多少种不同的子序列。
解题方法
动态规划
周赛的第四题，不会做，还是因为我的动态规划太弱了。。
瞻仰一下寒神的做法吧，膜拜！[C++/Java/Python] 4 lines O(N) Time, O(1) Spaceopen in new window。
使用一个endswith[26]数组，保存的是有多少个子序列以i结尾。则，当前总共有N = sum(endswith)个不同的子序列，当我们新增加一个字符c时，相当于在以前每个结尾的位置后面又增添了一个新的字符，所以现在有了N个以c结尾的不同的子序列了。
所以，我们遍历一遍s，更新的方式是end[c] = sum(end) + 1。加一是因为c本身也是一个子序列。
比如举个例子。
Input: "aba"
Current parsed: "ab"
endswith 'a': ["a"]
endswith 'b': ["ab","b"]
"a" -> "aa"
"ab" -> "aba"
"b" -> "ba"
"" -> "a"
endswith 'a': ["aa","aba","ba","a"]
endswith 'b': ["ab","b"]
result: 6
时间复杂度是O(26N)，空间复杂度是O(1)。
class Solution(object):
def distinctSubseqII(self, S):
"""
:type S: str
:rtype: int
"""
nums = [0] * 26
for s in S:
nums[ord(s) - ord("a")] = (sum(nums) + 1) % (10 ** 9 + 7)
return sum(nums) % (10 ** 9 + 7)
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