Method: longestSubstring

Input: Input string str

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1. Initialize all elements of lastPos to -1

2. startInd = 0

3. len = 0

4. max = 0

5. i = 0

6. For each character x in str at position i

    {

        a. if startInd <= lastPos[x]

            i.   startInd = lastPos[x]+1

            ii.  len = i-startInd + 1

            iii. lastPos[x] = i

        b. if startInd > lastPos[x] 

            i.   lastPos[x] = i

            ii.  len++

            iii. if (len>max) max = len

    }

7. return max

}