<p>You are given a string <code>s</code>.</p>

<p>A split is called <strong>good</strong> if you can split <code>s</code> into two non-empty strings <code>s<sub>left</sub></code> and <code>s<sub>right</sub></code> where their concatenation is equal to <code>s</code> (i.e., <code>s<sub>left</sub> + s<sub>right</sub> = s</code>) and the number of distinct letters in <code>s<sub>left</sub></code> and <code>s<sub>right</sub></code> is the same.</p>

<p>Return <em>the number of <strong>good splits</strong> you can make in <code>s</code></em>.</p>

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

<pre>
<strong>Input:</strong> s = &quot;aacaba&quot;
<strong>Output:</strong> 2
<strong>Explanation:</strong> There are 5 ways to split <code>&quot;aacaba&quot;</code> and 2 of them are good. 
(&quot;a&quot;, &quot;acaba&quot;) Left string and right string contains 1 and 3 different letters respectively.
(&quot;aa&quot;, &quot;caba&quot;) Left string and right string contains 1 and 3 different letters respectively.
(&quot;aac&quot;, &quot;aba&quot;) Left string and right string contains 2 and 2 different letters respectively (good split).
(&quot;aaca&quot;, &quot;ba&quot;) Left string and right string contains 2 and 2 different letters respectively (good split).
(&quot;aacab&quot;, &quot;a&quot;) Left string and right string contains 3 and 1 different letters respectively.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;abcd&quot;
<strong>Output:</strong> 1
<strong>Explanation:</strong> Split the string as follows (&quot;ab&quot;, &quot;cd&quot;).
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 10<sup>5</sup></code></li>
	<li><code>s</code> consists of only lowercase English letters.</li>
</ul>

class Solution {
public:
    int numSplits(string s) {
        int startLeft = -1;
        int i = 0;
        int startRight = s.size()-1;
        int j = s.size() - 1;
        unordered_set<char> charsLeft = {s[i]};
        unordered_set<char> charsRight = {s[j]};
        while (i<j) {
            if (charsLeft.size() < charsRight.size()) {
                i++;
                charsLeft.insert(s[i]);
                startLeft = -1;
            } else if (charsLeft.size() > charsRight.size()) {
                j--;
                charsRight.insert(s[j]);
                startLeft = -1;
            } else {
                if (startLeft == -1) {
                    startLeft = i;
                    startRight = j;
                }
                while (j>0 && charsRight.find(s[j-1]) != NULL) {
                    if (charsLeft.find(s[j-1]) == NULL) {
                        startRight = j-1;
                    }
                    j--;
                }
                while (i<s.size()-1 && charsLeft.find(s[i+1]) != NULL) {
                    if (charsRight.find(s[i+1]) == NULL) {
                        startLeft = i+1;
                    }
                    i++;
                }
                if (i>=j-1)
                    break;
                i++;
                charsLeft.insert(s[i]);
                startLeft = -1;
            }
        }

        if (startLeft == -1)
            return 0;
        return startRight - startLeft;
    }
};
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