leetcode414 C++ 4ms 第三大的数字

class Solution {
public:
    void siftdown(vector<int>& nums, int e, int begin, int end){
    int i = begin;
    int j = 2*begin + 1;
    while(j<end){
        if(j+1 < end && nums[j+1] > nums[j]){
            j++;
        }
        if(e > nums[j]){
            break;
        }
        nums[i] = nums[j];
        i = j;
        j = 2*i + 1;
    }
    nums[i] = e;
}

int thirdMax(vector<int>& nums) {
    if(nums.empty()){
        return 0;
    }
    if(nums.size() <=2){
        return *std::max_element(nums.begin(), nums.end());
    }
    int ma = *std::max_element(nums.begin(), nums.end());
    int last=-1000;
    int topk = 3;
    auto end = (int)nums.size();
    for(auto k = end/2; k>=0;k--){
        siftdown(nums, nums[k], k, end);
    }
    auto n = nums;
    for(int k = end-1;k>=0;k--){
        int e = nums[k];
        if(last != nums[0]){
            topk--;
            last = nums[0];
        }
        if(topk==0){
            return nums[0];
        }
        siftdown(nums, e, 0, k);
    }
    return ma;
}
};