<p>Given a string <code>s</code> and an array of strings <code>words</code>, return <em>the number of</em> <code>words[i]</code> <em>that is a subsequence of</em> <code>s</code>.</p>

<p>A <strong>subsequence</strong> of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.</p>

<ul>
	<li>For example, <code>&quot;ace&quot;</code> is a subsequence of <code>&quot;abcde&quot;</code>.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;abcde&quot;, words = [&quot;a&quot;,&quot;bb&quot;,&quot;acd&quot;,&quot;ace&quot;]
<strong>Output:</strong> 3
<strong>Explanation:</strong> There are three strings in words that are a subsequence of s: &quot;a&quot;, &quot;acd&quot;, &quot;ace&quot;.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;dsahjpjauf&quot;, words = [&quot;ahjpjau&quot;,&quot;ja&quot;,&quot;ahbwzgqnuk&quot;,&quot;tnmlanowax&quot;]
<strong>Output:</strong> 2
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 5 * 10<sup>4</sup></code></li>
	<li><code>1 &lt;= words.length &lt;= 5000</code></li>
	<li><code>1 &lt;= words[i].length &lt;= 50</code></li>
	<li><code>s</code> and <code>words[i]</code> consist of only lowercase English letters.</li>
</ul>

class Solution {
    bool isSubsequence(string &s, unordered_map<char, vector<int>>& indices, string &word, unordered_map<string, bool>& memo) {
        if (memo.find(word) != memo.end()) return memo[word];
        
        int m = word.length();
        int n = s.length();
        if (m > n) return memo[word] = false;

        int i = 0;
        int prev = -1;
        while (i < m) {
            if (indices.find(word[i]) == indices.end()) return memo[word] = false;
            vector<int>& ind = indices[word[i]];
            auto it = upper_bound(begin(ind), end(ind), prev);
            if (it == ind.end()) return memo[word] = false;
            prev = *it;
            i++;
        }
        return memo[word] = true;
    }

public:
    int numMatchingSubseq(string s, vector<string>& words) {
        int cnt = 0;
        unordered_map<char, vector<int>> indices;
        unordered_map<string, bool> memo;

        for (int i = 0; i < s.length(); i++) {
            indices[s[i]].push_back(i);
        }

        for (auto& word : words) {
            if (isSubsequence(s, indices, word, memo)) cnt++;
        }

        return cnt;
    }
};

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