Minimum Window Subsequence
<p>Given strings <code>s1</code> and <code>s2</code>, return <em>the minimum contiguous&nbsp;substring part of </em><code>s1</code><em>, so that </em><code>s2</code><em> is a subsequence of the part</em>.</p>

<p>If there is no such window in <code>s1</code> that covers all characters in <code>s2</code>, return the empty string <code>&quot;&quot;</code>. If there are multiple such minimum-length windows, return the one with the <strong>left-most starting index</strong>.</p>

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

<pre>
<strong>Input:</strong> s1 = &quot;abcdebdde&quot;, s2 = &quot;bde&quot;
<strong>Output:</strong> &quot;bcde&quot;
<strong>Explanation:</strong> 
&quot;bcde&quot; is the answer because it occurs before &quot;bdde&quot; which has the same length.
&quot;deb&quot; is not a smaller window because the elements of s2 in the window must occur in order.
</pre>

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

<pre>
<strong>Input:</strong> s1 = &quot;jmeqksfrsdcmsiwvaovztaqenprpvnbstl&quot;, s2 = &quot;u&quot;
<strong>Output:</strong> &quot;&quot;
</pre>

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

<ul>
	<li><code>1 &lt;= s1.length &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>1 &lt;= s2.length &lt;= 100</code></li>
	<li><code>s1</code> and <code>s2</code> consist of lowercase English letters.</li>
</ul>
String; Dynamic Programming; Sliding Window
class Solution {
public:
    string minWindow(string s1, string s2) {
        int m = s1.size(), n = s2.size();
        int f[m + 1][n + 1];
        memset(f, 0, sizeof(f));
        for (int i = 1; i <= m; ++i) {
            for (int j = 1; j <= n; ++j) {
                if (s1[i - 1] == s2[j - 1]) {
                    f[i][j] = j == 1 ? i : f[i - 1][j - 1];
                } else {
                    f[i][j] = f[i - 1][j];
                }
            }
        }
        int p = 0, k = m + 1;
        for (int i = 1; i <= m; ++i) {
            if (s1[i - 1] == s2[n - 1] && f[i][n]) {
                int j = f[i][n] - 1;
                if (i - j < k) {
                    k = i - j;
                    p = j;
                }
            }
        }
        return k > m ? "" : s1.substr(p, k);
    }
};