[day_060423] convert string to integer

Convert a string to an integer

Title source

Cattle from ： Convert a string to an integer

Title Description

Convert a string to an integer , Library functions that require that integers cannot be converted using strings . Values for 0 Or if the string is not a valid value, it returns 0

Data range ： The string length satisfies 0 \le n \le 100 \0≤n≤100
Advanced ： Spatial complexity O(1) \O(1) , Time complexity O(n) \O(n)

Be careful ：
① Any symbol... May appear in the string , Occurrence Division +/- Output directly when other symbols 0
② Possible in string +/- And can only appear at the beginning of the string .

Input description

Enter a string , Including alphanumeric symbols , Can be null

Output description

If it is a legal numeric expression, it returns the number , Otherwise return to 0

Example 1

Input

“[phone]”

Return value

2147483647

Example 2

Input

“1a33”

Return value

0

Thought analysis

• String to number , use ASCII Value minus ’0’ Of ASCII You can get
• Handle the beginning and end of the string , If there is ’+‘’-' Number , marked
• Traversal string , It is similar to the idea of getting numbers on each bit of a multi digit number , It can be understood as its inverse process , Every time before result Value times 10, Add the value of the current number as a bit

Code display

class Solution {
    
public:
    int StrToInt(string str) {
    
        if (str.size() == 0)
        {
    
            return 0;
        }
        vector<int> v;
        int flag = 1;
        int result = 0;
        int i = 0;
        // Judge first 
        if (!(isdigit(str[0])))
        {
    
            // If the first sign is given i+1, Start traversing from the second position 
            i++;
            if (str[0] == '-')
            {
    
                flag = -1;
            }
            else if (str[0] == '+')
            {
    
                flag = 1;
            }
            else
            {
    
                return 0;
            }
        }
        while (i < str.size())
        {
    
            if (!(isdigit(str[i])))
            {
    
                return 0;
            }
            // Use characters ASCII Subtract characters from numbers '0' Of ASCII value , Get the corresponding integer number 
            result = result * 10 + str[i] - '0';
            i++;
        }
        return result * flag;
    }
};

summary

• isdigit function , Determine whether a character is a numeric character
• push_back(),vector Tail connector ,vector No head connector , It can be used insert To replace