# C solution, 3ms

• ``````int lengthOfLongestSubstring(char* s) {
int maxLen = 0, prevMaxLen = 0, prevIndexOf[128];

bzero(prevIndexOf, sizeof(prevIndexOf));
for (char *c = s; *c; c++) {
int idx = c - s + 1;   // Add 1 to the index since prevIndexOf is initialized with zeros
int candidateLen = idx - prevIndexOf[*c];
int curMaxLen = (++prevMaxLen < candidateLen) ? prevMaxLen : candidateLen;
prevIndexOf[*c] = idx;

if (curMaxLen > maxLen) {
maxLen = curMaxLen;
}
prevMaxLen = curMaxLen;
}

return maxLen;
}
``````

• wow ! amazing algorithm!!!
could you tell me how you came up this algorithm?plz

• Thanks @JasonZWJ

This was inspired by Kadane's algorithm for the maximal subarray problem.
The basic idea is that the length of the longest substring ending at position `i` is the lesser of

• length(longest substring ending at position `i - 1`) + 1
• `i - prevIndexOf[*c]` i.e. the number of characters since the previous occurrence of the character at position `i`

So we can use the longest substring length at each position and to compute the length at the succeeding position. It's convenient to initialize the `prevIndexOf` array with zeros, so the index computations have to be offset by one to account for that.

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