[LeetCode] 115. Distinct Subsequences

ProblemGiven a string S and a string T, count the number of distinct subsequences of S which equals T.A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie, “ACE” is a subsequence of “ABCDE” while “AEC” is not).Example 1:Input: S = “rabbbit”, T = “rabbit"Output: 3Explanation:As shown below, there are 3 ways you can generate “rabbit” from S.(The caret symbol ^ means the chosen letters)rabbbit^^^^ ^^rabbbit^^ ^^^^rabbbit^^^ ^^^Example 2:Input: S = “babgbag”, T = “bag"Output: 5Explanation:As shown below, there are 5 ways you can generate “bag” from S.(The caret symbol ^ means the chosen letters)babgbag^^ ^babgbag^^ ^babgbag^ ^^babgbag ^ ^^babgbag ^^^Solutionclass Solution { public int numDistinct(String s, String t) { int m = s.length(), n = t.length(); if (m < n) return 0; int[][] dp = new int[m+1][n+1]; for (int i = 0; i <= m; i++) dp[i][0] = 1; for (int j = 0; j < n; j++) { for (int i = 0; i < m; i++) { if (s.charAt(i) == t.charAt(j)) dp[i+1][j+1] = dp[i][j] + dp[i][j+1]; else dp[i+1][j+1] = dp[i][j+1]; } } return dp[m][n]; }}