Is Subsequence

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392.Is Subsequence

Given two strings s and t, return true if s is a subsequence of t, or false otherwise.

A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., "ace" is a subsequence of "abcde" while "aec" is not).

Example 1:

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Input: s = "abc", t = "ahbgdc"
Output: true

Example 2:

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Input: s = "axc", t = "ahbgdc"
Output: false

双指针：

时间复杂度：O(n+m)

空间复杂度：O(1)

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class Solution {
public:
    bool isSubsequence(string s, string t) {
        int n = s.length(), m = t.length();
        int i = 0, j = 0;
        while (i < n && j < m) {
            if (s[i] == t[j]) {
                i++;
            }
            j++;
        }
        return i == n;
    }
};

动态规划：

时间复杂度：O(m*S+n)，S为字符集大小

空间复杂度：O(m*s)

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class Solution {
public:
    bool isSubsequence(string s, string t) {
        int n = s.size(), m = t.size();

        vector<vector<int> > f(m + 1, vector<int>(26, 0));
        for (int i = 0; i < 26; i++) {
            f[m][i] = m;
        }

        for (int i = m - 1; i >= 0; i--) {
            for (int j = 0; j < 26; j++) {
                if (t[i] == j + 'a')
                    f[i][j] = i;
                else
                    f[i][j] = f[i + 1][j];
            }
        }
        int add = 0;
        for (int i = 0; i < n; i++) {
            if (f[add][s[i] - 'a'] == m) {
                return false;
            }
            add = f[add][s[i] - 'a'] + 1;
        }
        return true;
    }
};