leetcode 215: Kth Largest Element in an Array

Kth Largest Element in an Array

Total Accepted: 3277 Total Submissions: 12235

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.

[思路]

算法书中,专门的一节,  用到了和quicksort同样的partition, 算法复杂度是O(n). 直接sort是O(n*log(n))

[CODE]

public class Solution {    //test case [2, 1], 1    public int findKthLargest(int[] nums, int k) {        return findK(nums, nums.length-k, 0, nums.length-1);    }        private int findK(int[] nums, int k, int i, int j) {        if(i>=j) return nums[i];        int m = partition(nums, i, j);        if(m==k) return nums[m];        else if(m<k) {            return findK(nums, k, m+1, j);        } else {            return findK(nums, k, i, m-1);        }    }        private int partition(int[] nums, int i, int j) {        int x = nums[i];        int m = i;        int n = i+1;                while(n<=j){            if(nums[n]<x) {                swap(nums, ++m, n);            }            ++n;        }        swap(nums,i, m);        return m;    }        private void swap(int[] nums, int i, int j) {        int temp = nums[i];        nums[i] = nums[j];        nums[j] = temp;    }}