LeetCode "Distinct Subsequences"

A classic and representative DP problem. To be revised later. Quite a few interesting details to think about.

class Solution {
public:
    int numDistinct(string S, string T) {
        int lenS = S.length();
        int lenT = T.length();
        if (lenS < lenT) return 0;
        if (lenS == lenT)
        {
            if (S.compare(T) == 0) return 1;
            else return 0;
        }

        vector<int> dp(lenT + 1, 0);
        dp[0] = 1;　　　　　　　　　　　　// Point 1

        //    S: i, T: j
        //    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]
        for (int i = 1; i <= lenS; i ++)
        for (int j = lenT; j >= 1; j--) // Point 2
        {
            if(i < j) continue;
            if (S[i - 1] == T[j - 1])
                dp[j] += dp[j - 1];　　// Point 3
        }
        return dp[lenT];
    }
};