word consists of only lowercase and uppercase English letters.

Solution

Method 1 – Index Tracking and Set Check

Intuition

A letter is special if both its lowercase and uppercase forms appear, and all lowercase occurrences come before the first uppercase occurrence. We can track the last index of each lowercase and the first index of each uppercase, then check the order.

Approach

For each letter 'a' to 'z', track:
- The last index of its lowercase occurrence (lastLower)
- The first index of its uppercase occurrence (firstUpper)
Iterate through the string and update these indices.
For each letter, if both indices are set and all lowercase indices are before the first uppercase, count it as special.

Code

[{"language":"C++","code":"class Solution {\npublic:\n    int numberOfSpecialChars(string word) {\n        vector<int> lastLower(26, -1), firstUpper(26, -1);\n        int n = word.size();\n        for (int i = 0; i < n; ++i) {\n            char c = word[i];\n            if (islower(c)) lastLower[c - 'a'] = i;\n            else if (isupper(c) && firstUpper[c - 'A'] == -1) firstUpper[c - 'A'] = i;\n        }\n        int ans = 0;\n        for (int i = 0; i < 26; ++i) {\n            if (lastLower[i] != -1 && firstUpper[i] != -1 && lastLower[i] < firstUpper[i]) ++ans;\n        }\n        return ans;\n    }\n};","lang":"cpp","html":"<pre class=\"shiki shiki-themes github-light github-dark\" style=\"background-color:#fff;--shiki-dark-bg:#24292e;color:#24292e;--shiki-dark:#e1e4e8\" tabindex=\"0\"><code><span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">class</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> Solution</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\"> {</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">public:</span></span>\n<span class=\"line\"><span style=\"color:#D73A49;--shiki-dark:#F97583\">    int</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> numberOfSpecialChars</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">(</span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\">string</span><span style=\"color:#E36209;--shiki-dark:#FFAB70\"> word</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">) {</span></span>\n<span class=\"line\"><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">        vector</span><span style=\"color:#D73A49;--shiki-dark:#F97583\">&#x3C;int></span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\"> lastLower</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">(</span><span style=\"color:#005CC5;--shiki-dark:#79B8FF\">26</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">, </span><span style=\"color:#D73A49;--shiki-dark:#F97583\">-</span><span style=\"color:#005CC5;--shiki-dark:#79B8FF\">1</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">), </span><span style=\"color:#6F42C1;--shiki-dark:#B392F0\">firstUpper</span><span style=\"color:#24292E;--shiki-dark:#E1E4E8\">(</span><span style=\"color:#005CC5;--shiki-dark:#79B8FF\">26</span><span style=\"color:#24292E;--sh

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