Case-specific Sorting of Strings

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Intuition

Sort string keeping uppercase in their positions and lowercase in their positions separately.

Algorithm

1Extract uppercase letters and positions, extract lowercase letters and positions. Sort each group. Re-place.

Common Pitfalls

•Two separate sorted sequences. Map back to original case-specific positions. O(n log n).

Case-specific Sorting of Strings.java

Java

// Approach: Separate uppercase and lowercase characters, sort each group, then merge back in original case positions.
// Time: O(n log n) Space: O(n)
class Solution {
    public static String caseSort(String s) {
        int[] upper = new int[26];
        int[] lower = new int[26];
        for (char a : s.toCharArray()) {
            if (a >= 'a' && a <= 'z')
                lower[a - 'a']++;
            else
                upper[a - 'A']++;

        }
        int i = 0;
        int j = 0;
        StringBuilder sb = new StringBuilder();
        for (int k = 0; k < s.length(); k++) {
            char ch = s.charAt(k);
            if (ch >= 'a' && ch <= 'z') {
                while (lower[i] == 0)
                    i++;

                sb.append((char) ('a' + i));
                lower[i]--;
            } else {
                while (upper[j] == 0)
                    j++;

                sb.append((char) ('A' + j));
                upper[j]--;
            }
        }
        
        return sb.toString();
    }
}

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