问题描述:
Given a string S and a string T, count the number of distinct subsequences of T in S.
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).
Here is an example:
S = “rabbbit”, T = “rabbit”
Return 3.
分析:这个问题问的实在不清楚,实际上是问s中有多少个子序列等于t。
那这个题看上去非常像0、1背包问题,对于字符串中的每一个字符都有两种选择,1是选,1是不选。另外递归+剪枝构成如下代码,结果TLE
代码如下:TLE
public class Solution {
int count = ;
int depth;
String t;
int tlen;
public int numDistinct(String s, String t) {
StringBuilder builder = new StringBuilder();
depth = s.length();
this.t = t;
tlen = t.length();
solve(s, ,builder);
return count;
}
private void solve(String s,int level,StringBuilder builder){
if(level == depth){
return;
}
char ch = s.charAt(level);
builder.append(ch);
int size = builder.length();
if(size<=tlen && ch == t.charAt(size-)) {
if(size==tlen) {
count++;
}
solve(s, level + , builder);
}
builder.deleteCharAt(size-);
solve(s,level+,builder);
}
}
然后想到是不是可以用DP算法。
令字符串s的长度为slen,令字符串t的长度为tlen,那么最后的问题是slen的s中删除若个字符可否变成tlen的t,有几种?
那么子问题就是长度为i的s的子串可否变成长度为j的t的子串?有几种。
那么递推关系式就变成了
dp[i][j] = chars[i]==chars[j]?dp[i-1][j-1]+dp[i-1][j] : dp[i-1][j];
于是代码如下:300ms
public class Solution {
public int numDistinct(String s, String t) {
int row = s.length();
int col = t.length();
if(row<= || col<=)
return ;
int[][] dp = new int[row][col];
char[] chars = s.toCharArray();
char[] chart = t.toCharArray();
dp[][] = chars[]==chart[]?:;//init first row
//init first col
for(int i = ;i<row;i++){
dp[i][] = dp[i-][];
if(chars[i] == chart[])
dp[i][] += ;
}
for(int i = ;i<row;i++){
for(int j = ;j<col;j++){
dp[i][j] = dp[i-][j];
if(chars[i]==chart[j])
dp[i][j] += dp[i-][j-];
}
}
return dp[row-][col-];
}
}