DSA

String to Integer (atoi)

Strings. Time O(n), Space O(1).

August 8, 2026

Practice Link

Approach#

Process the string in four sequential steps: (1) skip leading whitespace, (2) detect an optional + or - sign, (3) skip leading zeros, (4) accumulate digit characters into a number. After each digit is added, check for overflow against INT_MAX / INT_MIN and clamp immediately. Stop at the first non-digit character. All operations are a single left-to-right scan making it O(n) time and O(1) space.

cpp

class Solution {
public:
    int myAtoi(string s) {
        if(s.length()==0)
            return 0;

        //ignore trailing whitespaces
        int start =0;
        while (start < s.length() && s[start] == ' ')
            start++;

        //check sign
        int sign = 1;
        if (start < s.length() && (s[start] == '+' || s[start] == '-')) {
            if (s[start] == '-') sign = -1;
            start++;
        }

        //ignore trailing zereos
        while (start < s.length() && s[start] == '0')
            start++;

        int ind = start;
        while(ind<s.length() && s[ind]>='0' && s[ind]<='9')
            ind++;

        if(start==ind)
            return 0;

        long long num=0;
        for(int i=start;i<ind;i++)
        {
            int digit = s[i]-'0';
            num = num*10 + digit;
            if(sign==-1 && -num < INT32_MIN)
                return INT32_MIN;
            if(sign == 1 && num > INT32_MAX)
                return INT32_MAX;
        }

        return (sign)*num;
    }
};

Time Complexity: O(n)

Space Complexity: O(1)

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Approach#

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