Leetcode 115. Distinct Subsequences

https://leetcode.com/problems/distinct-subsequences/

DP. While DP also has ordinary solution and enhanced solution.

Solution 1:

# T:O(m*n) S:O(m*n)
class Solution:
    # @return an integer
    # @dp
    # dp[i][j] means how many first j of T is sub of first i of S.
    def numDistinct(self, S, T):
        dp = [[0 for i in range(len(T)+1)] for j in range(len(S)+1)]
        for j in range(len(S)+1):
            dp[j][0] = 1
        for i in range(1, len(S)+1):
            for j in range(1, min(i+1, len(T)+1)):
                if S[i-1] == T[j-1]:
                    dp[i][j] = dp[i-1][j] + dp[i-1][j-1]
                else:
                    dp[i][j] = dp[i-1][j]
        return dp[len(S)][len(T)]

Run Time: 252 ms

Solution 2:

# T:O(n^2) S:O(n)
class Solution:
    # @return an integer
    def numDistinct(self, S, T):
        ways = [0 for _ in xrange(len(T) + 1)]
        ways[0] = 1
        for S_char in S:
            for j, T_char in reversed(list(enumerate(T))):
                if S_char == T_char:
                    ways[j + 1] += ways[j]
        return ways[len(T)]

Run Time: 116 ms