[Algorithm] 10. Reverse Integer

Description

Given a 32-bit signed integer, reverse digits of an integer.

Example

Example 1:

Input: 123
Output: 321

Example 2:

Input: -123
Output: -321

Example 3:

Input: 120
Output: 21

Note

Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231,  231 − 1]. For the purpose of this problem, assume that your function returns 0 when the reversed integer overflows.

Solution

    int reverse(int x) {
        bool minus_flag = false;
        bool zeroIgnore = true;
        int result=0;
        stack<char> numPool;
        
        if( x < 0 ){
            x = -x;
            minus_flag = true;
        }
        
        if( x/10 == 0 ){
            return minus_flag ? -x : x;
        } 
        
        // Put each number into Stack container.
        while(x){
            if ( x % 10 ) {
                numPool.push(x % 10);
                zeroIgnore = false;
            }else{
                if ( !zeroIgnore )
                    numPool.push(0);
            }
            x = x/10;
        }
        
        // Calculate total sum.
        int cnt=numPool.size();
        for(int i=0; i<cnt; i++)
        {
            result+=numPool.top() * pow(10, i);
            // Judge if the result overflows.
            if (result < numPool.top() * pow(10, i) ) {
                return 0;
            }
            numPool.pop();
        }
        
        return minus_flag ? -result : result;
    }