Kth Largest Element in an Array

Leetcode

題意：

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.

解題思路：

使用 quick select來達到 O(N)的時間複雜度，不斷的用pivot把陣列切兩半，其程式碼如下：

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        int len = nums.length;
        if (k > len) {
            return 0;
        }
        return quickSelect(nums, 0, len - 1, len - k);
    }

    private int quickSelect(int[] nums, int left, int right, int k) {
        int pivot = right;
        int num = nums[pivot];
        int low = left;
        int high = right;

        while (low < high) {
            //從low開始找到一個比pivot大的數
            while (low < high && nums[low] < num) {
                low++;
            }
            // 從high開始找到一個比pivot小的數
            // 設>=表示忽略pivot
            while (low < high && nums[high] >= num) {
                high--;
            }
            swap(nums, low, high);
        }
        // 交換pivot與目前high指向的位置
        // 因pivot選最後一位，因此必須選一個比pivot大的數來作交換
        // 此時low指針已high指針已重疊，即該數大於pivot，直接作交換
        swap(nums, low, pivot);

        if (low == k) {
            return nums[low];
        }
        if (low > k) {
            return quickSelect(nums, left, low - 1, k);
        } else {
            //因為陣列的index都還在，所以直接丟k進去即可
            return quickSelect(nums, low + 1, right, k);
        }

    }

    private void swap(int[] nums, int idxA, int idxB) {
        int temp = nums[idxA];
        nums[idxA] = nums[idxB];
        nums[idxB] = temp;
    }
}

updated 2015.12.1

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        if (nums == null || k == 0) {
            return 0;
        }
        return quickSelect(nums, 0, nums.length - 1, k);
    }

    private int quickSelect(int[] nums, int start, int end, int k) {
        int pivot = end;
        int left = start;
        int right = end - 1;

        while (left <= right) {
            if (nums[left] > nums[pivot]) {
                swap(nums, left, right);
                right--;
            } else {
                left++;
            }
        }

        swap(nums, left, pivot);
        int rank = nums.length - left;
        if (rank == k) {
            return nums[left];
        }
        if (rank > k) {
            return quickSelect(nums, left + 1, end, k);
        } else {
            return quickSelect(nums, start, left - 1, k);
        }
    }

    private void swap(int[] nums, int a, int b) {
        int temp = nums[a];
        nums[a] = nums[b];
        nums[b] = temp;
    }
}

Reference

1. https://leetcode.com/discuss/59628/java-concise-quickselect