left pointer
Scans rightward from the start, skipping every consonant until it finds a vowel or meets the right pointer.
Swap only the vowels, leave the frame intact.
A typographer mirrors only the vowel glyphs in a display string while leaving consonants fixed — reflecting 'hello' to 'holle'. Two pointers converge from opposite ends, each skipping consonants. Only when both land on a vowel do they swap. The consonant skeleton stays intact while the vowels reflect across it. O(n) time. O(n) space for the character array.
Watch two pointers converge and swap
The two-pointer vowel invariant
The left pointer only advances past consonants; the right pointer only retreats past consonants. A swap happens exactly when both pointers sit on vowels — guaranteeing every vowel finds its mirror partner.
right pointer
Scans leftward from the end, skipping every consonant until it finds a vowel or meets the left pointer.
vowel swap
When both pointers rest on vowels, the characters swap. Both pointers then step inward and the search continues.
Convert, converge, swap, rebuild
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1
Convert the string to a char array
Strings are immutable, so copy the characters into a mutable array to allow in-place swaps.
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2
Initialize two pointers
Set
left = 0andright = length − 1. They will walk toward each other. -
3
Skip consonants, swap vowels
While
left < right: skip left if it's a consonant, skip right if it's a consonant, otherwise swap both characters and advance both pointers inward. -
4
Return the new string
Join the modified char array back into a string. All vowels are now reversed; consonants remain in place.
Where the pointers reveal the structure
No vowels at all
Both pointers race past consonants until they cross. No swap ever occurs — the string returns unchanged. The algorithm gracefully handles consonant-only inputs like "rhythm."
All vowels
Every character is a vowel, so neither pointer ever skips. The string fully reverses — identical to reversing the whole string. The algorithm performs ⌊n/2⌋ swaps.
Palindrome vowels
When the vowel sequence reads the same forwards and backwards, swaps still happen but produce the same characters. The output equals the input — e.g., "racecar" stays "racecar."