Leetcode Largest Number

Problem

Given a list of non negative integers, arrange them such that they form the largest number.

For example, given [3, 30, 34, 5, 9], the largest formed number is 9534330.

Note: The result may be very large, so you need to return a string instead of an integer.

Idea: After observing the array and the 

returned result, here we need to think 

for example, when given num1 = 5, 

num2 = 34, why we put num1 before num2. 

 The answer is simple, that is because

 num1 + num2 (534)  >  num2 + num1 (345). 

  Therefore, we need to first sort the

 array such that each adjacent two 

numbers (indicate them by n1 and n2,

  and assume that n1 is before n2 in

 the sorted array) satisfy the rule 

that  n1 + n2 > n2 + n1.  

So we need to first rewrite the Comparator.

Code:

public class Solution {
	class MyComparator implements Comparator<String> {
		public int compare (String s1, String s2) {
			String str1 = s1 + s2;
			String str2 = s2 + s2;
			return str2.compareTo(str1);
		}
	}
	public String largestNumber(int[] num) {
		String[] strArray = new String[num.length];
		for (int i = 0; i < num.length; i++) {
			strArray[i] = String.valueOf(num[i]);
		}
		Comparator<String> comparator = new MyComparator();
		Arrays.sort(strArray, comparator);
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < num.length; i++) {
			sb.append(strArray[i]);
		} 
		return sb.charAt(0) == '0' ? "0" : sb.toString();
	}

}