Minimum Recolors to Get K Consecutive Black Blocks
<p>You are given a <strong>0-indexed</strong> string <code>blocks</code> of length <code>n</code>, where <code>blocks[i]</code> is either <code>&#39;W&#39;</code> or <code>&#39;B&#39;</code>, representing the color of the <code>i<sup>th</sup></code> block. The characters <code>&#39;W&#39;</code> and <code>&#39;B&#39;</code> denote the colors white and black, respectively.</p>

<p>You are also given an integer <code>k</code>, which is the desired number of <strong>consecutive</strong> black blocks.</p>

<p>In one operation, you can <strong>recolor</strong> a white block such that it becomes a black block.</p>

<p>Return<em> the <strong>minimum</strong> number of operations needed such that there is at least <strong>one</strong> occurrence of </em><code>k</code><em> consecutive black blocks.</em></p>

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

<pre>
<strong>Input:</strong> blocks = &quot;WBBWWBBWBW&quot;, k = 7
<strong>Output:</strong> 3
<strong>Explanation:</strong>
One way to achieve 7 consecutive black blocks is to recolor the 0th, 3rd, and 4th blocks
so that blocks = &quot;BBBBBBBWBW&quot;. 
It can be shown that there is no way to achieve 7 consecutive black blocks in less than 3 operations.
Therefore, we return 3.
</pre>

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

<pre>
<strong>Input:</strong> blocks = &quot;WBWBBBW&quot;, k = 2
<strong>Output:</strong> 0
<strong>Explanation:</strong>
No changes need to be made, since 2 consecutive black blocks already exist.
Therefore, we return 0.
</pre>

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

<ul>
	<li><code>n == blocks.length</code></li>
	<li><code>1 &lt;= n &lt;= 100</code></li>
	<li><code>blocks[i]</code> is either <code>&#39;W&#39;</code> or <code>&#39;B&#39;</code>.</li>
	<li><code>1 &lt;= k &lt;= n</code></li>
</ul>
String; Sliding Window
class Solution {
    public int minimumRecolors(String blocks, int k) {
        int cnt = 0;
        for (int i = 0; i < k; ++i) {
            cnt += blocks.charAt(i) == 'W' ? 1 : 0;
        }
        int ans = cnt;
        for (int i = k; i < blocks.length(); ++i) {
            cnt += blocks.charAt(i) == 'W' ? 1 : 0;
            cnt -= blocks.charAt(i - k) == 'W' ? 1 : 0;
            ans = Math.min(ans, cnt);
        }
        return ans;
    }
}