LeetCode 215 : Kth Largest Element in an Array

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note: 
You may assume k is always valid, 1 ≤ k ≤ array's length.

最简单的想法：先排序，然后倒数k个元素就是

时间复杂度：O(nlogn)

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        Arrays.sort(nums);
        return nums[nums.length-k];
    }
}

优化：quicksort 中 partition 的思想

就像quickselect一样，时间复杂度应该是 O(n)

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        return helper(nums, 0, nums.length-1, k);
    }
    
    public int helper(int[] nums, int lo, int hi, int k) {
        // if (lo == hi)
        //     return nums[lo];
            
        int partition = nums[lo];
        int j = hi;
        for (int i = lo+1; i <= j;) {
            while (i <= hi && nums[i] < partition) i++;
            while (j >= lo && nums[j] > partition) j--;
            if (i <= j) {
                swap(nums, i, j);
                i++; j--;
            }
        }
        swap(nums, j, lo);

        if (k == hi - j + 1)
            return nums[j];
        if (k < hi - j + 1)
            return helper(nums, j + 1, hi, k);
        else
            return helper(nums, lo, j - 1, k - (hi - j + 1));
    }
    
    public void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}

2015-10-21