https://leetcode.com/problems/largest-number-at-least-twice-of-others/description/

class Solution {public:    int dominantIndex(vector
     
      & nums) {        if (nums.size() == 0)    return -1;        int max1 = nums[0], max2, max1index = 0, max2index = -1;        for (int i = 1; i < nums.size(); i++) {            if (nums[i] >= max1) {      // we don't need to update max1 when it == nums[i] only for this problem                if (nums[i] > max1) {                    max2 = max1;                    max2index = max1index;                    max1 = nums[i];                    max1index = i;                }            }            else if (max2index < 0 || nums[i] > max2) {                max2 = nums[i];                max2index = i;            }        }        return (max2index < 0 || max1 >= 2 * max2) ? max1index : -1;    }};