[LintCode] 第k大元素

基于快速排序：

class Solution {
public:
    /*
     * param k : description of k
     * param nums : description of array and index 0 ~ n-1
     * return: description of return
     */
    int kthLargestElement(int k, vector<int> nums) {
        // write your code here
        int left = 0, right = nums.size() - 1;
        while (true) {
            int pos = partition(nums, left, right);
            if (pos == k - 1) return nums[pos];
            if (pos < k - 1) left = pos + 1;
            if (pos > k - 1) right = pos - 1;
        }
    }
private:
    int partition(vector<int>& nums, int left, int right) {
        int pivot = nums[left];
        int l = left + 1, r = right;
        while (l <= r) {
            if (nums[l] < pivot && nums[r] > pivot)
                swap(nums[l++], nums[r--]);
            if (nums[l] >= pivot) l++;
            if (nums[r] <= pivot) r--;
        }
        swap(nums[left], nums[r]);
        return r;
    }
};

 基于最大堆：

class Solution {
public:
    /*
     * param k : description of k
     * param nums : description of array and index 0 ~ n-1
     * return: description of return
     */
    inline int left(int idx) {
        return (idx << 1) + 1;
    }
    inline int right(int idx) {
        return (idx << 1) + 2;
    }
    void max_heapify(vector<int>& nums, int idx) {
        int largest = idx;
        int l = left(idx), r = right(idx);
        if (l < heap_size && nums[l] > nums[largest])
            largest = l;
        if (r < heap_size && nums[r] > nums[largest])
            largest = r;
        if (largest != idx) {
            swap(nums[idx], nums[largest]);
            max_heapify(nums, largest);
        }
    }
    void build_max_heap(vector<int>& nums) {
        heap_size = nums.size();
        for (int i = (heap_size >> 1) - 1; i >= 0; i--)
            max_heapify(nums, i);
    }
    int kthLargestElement(int k, vector<int> nums) {
        // write your code here
        build_max_heap(nums);
        for (int i = 0; i < k; i++) {
            swap(nums[0], nums[heap_size - 1]);
            heap_size--;
            max_heapify(nums, 0);
        }
        return nums[heap_size];
    }
private:
    int heap_size;
};