179. Largest Number

Problem:

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

Example 1:

Input: [10,2]
Output: "210"

Example 2:

Input: [3,30,34,5,9]
Output: "9534330"

Analysis:
Sort the array first. To determine which number comes first see below.
s1 = "30", s2 = "34"
s1 + s2 = "3034"
s2 + s1 = "3430"
s2 is ahead of s1, so the comparator in lambda expression is (s1, s2) -> (s2 + s1).compareTo(s1 + s2).

Solution:

class Solution {
    public String largestNumber(int[] nums) {
        if (nums == null || nums.length == 0)
            return "";
        String[] strs = new String[nums.length];
        for (int i = 0; i < nums.length; i++) {
            strs[i] = String.valueOf(nums[i]);
        }
        Arrays.sort(strs, ((a,b) -> (b + a).compareTo(a + b)));
        if (strs[0].charAt(0) == '0') return "0";
        StringBuilder sb = new StringBuilder();
        for (String s: strs) {
            sb.append(s);
        }

        return sb.toString();
    }
}