Leetcode 115. 不同的子序列

class Solution {
    //dp[i][j]表示s中前i个字符包含t中前j个字符的个数
    public int numDistinct(String s, String t) {
        int n1 = s.length();
        int n2 = t.length();
        if(n1 < n2) return 0;
        int[][] dp = new int[n1 + 1][n2 + 1];
        for(int i = 0; i <= n1; i++) dp[i][0] = 1;
        for(int i = 1; i <= n1; i++){
            for(int j = 1; j <= n2; j++){
                if(s.charAt(i - 1) == t.charAt(j - 1)){
                    dp[i][j] = dp[i - 1][j - 1] + dp[i - 1][j];
                }
                else{
                    dp[i][j] = dp[i - 1][j];
                }
            }
        }
        return dp[n1][n2];
    }
}