Largest Number

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

Notice

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

Example

Given [1, 20, 23, 4, 8], the largest formed number is8423201.

Analyse: to determine how to place a, b, we could combine a and b in string way and compare to_string(a) + to_string(b), to_string(b) + to_string(a).  Aftersorting, it could be 00000008 or 0000000, we need to check whether there is consecutive 0s in the highest index. Lambda is used to sort self-defined comparator. 

Runtime: 20ms.

class Solution {
public:
    /**
     *@param num: A list of non negative integers
     *@return: A string
     */
    string largestNumber(vector<int> &num) {
        // write your code here
        string result;
        if (num.empty()) return result;
        sort(num.begin(), num.end(), [](int a, int b) {
            // regard a, b as string, append b to a and a to b
            // compare a + b and b + a
            string ab = to_string(a) + to_string(b);
            string ba = to_string(b) + to_string(a);
            return ab < ba;
        });
        int i = num.size() - 1;
        while (i >= 0 && !num[i]) i--;
        while (i >= 0)
            result += to_string(num[i--]);
        return result.empty() ? "0" : result;
    }
};