way 1. while loop 224ms O(n^2)

public class Solution {

public int addDigits(int num) {

if( num < 10 ){ return num ; }

int digitSum= getDigitSum( num );

while(digitSum >=10 ){

digitSum = getDigitSum( digitSum );

}

return digitSum ;

}

private int getDigitSum( int num ){

int reminder=0;

while( num >= 10 ){

reminder += num%10;

num= num/10;

}

return num+reminder;

}

}

9/ 25 /2015

public class Solution {

public int addDigits(int num) {

if( num < 10 ){ return num; }

int temp=0 ;

while( num >=10 ){

temp+= num%10;

num/=10;

}

int rst= temp+ num;

if( rst >= 10 ){

rst= addDigits(rst );

}

return rst;

}

}

Discussion : https://leetcode.com/discuss/59588/my-simple-java-solution-use-loop-and-without-loop

Wiki: https://en.wikipedia.org/wiki/Digital_root

public class Solution {

public int addDigits(int num) {

int result= 0;

if( num < 10 ){ return num; }

while( num >=10 ){

result += num % 10 ;

num/=10;

}

return addDigits( result+num );

}

}

// amazing .....

public class Solution {

public int addDigits(int num) {

// Wiki

return num-9*((num-1)/9);

}

}