https://leetcode.com/problems/largest-number/

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.

1.比較函式不是直接比較兩個string的大小

因為30>3，但是3卻因為放在前面；所以因為比較字串組合起來的大小；

2.注意首位有0的情況

class Solution {
public:
	struct cmp{
		bool operator()(const string& a, const string& b)const{
			string ab = a + b;
			string ba = b + a;
			return ab > ba;
		}//注意這個比較函式
	};
	string largestNumber(vector<int>& nums) {
		int len = nums.size();
		if (len == 0){
			return "";
		}
		vector<string> strNums(len);
		for (int i = 0; i < len; i++){
			strNums[i] = to_string(nums[i]);
		}
		sort(strNums.begin(), strNums.end(), cmp());
		string res;
		for (int i = 0; i < len; i++){
			res.append(strNums[i]);
		}

		//[0,0]的情況
		while (res.size()>1 && res[0]=='0'){
			res = res.substr(1);
		}
		
		return res;
	}
};