80.中位数

描述

给定一个未排序的整数数组，找到其中位数。
中位数是排序后数组的中间值，如果数组的个数是偶数个，则返回排序后数组的第N/2个数。

样例

给出数组[4, 5, 1, 2, 3]， 返回 3
给出数组[7, 9, 4, 5]，返回 5

挑战

时间复杂度为O(n)

代码

public class Solution {
    /*
     * @param nums: A list of integers
     * @return: An integer denotes the middle number of the array
     */
    public int median(int[] nums) {
        if (nums == null) {
            return - 1;
        }
        
        return quickSelect(nums, 0, nums.length - 1, (nums.length + 1) / 2);
    }
    
    private int quickSelect(int[] nums, int start, int end, int k) {
        if (start >= end) {
            return nums[start];
        }
        
        int left = start;
        int right = end;
        int pivot = nums[start + (end - start) / 2];
        
        while (left <= right) {
            while (left <= right && nums[left] < pivot) {
                left++;
            }
            while (left <= right && nums[right] > pivot) {
                right--;
            }
            if (left <= right) {
                int temp = nums[left];
                nums[left] = nums[right];
                nums[right] = temp;
                left++;
                right--;
            }
        }
        
        if (start + k - 1 <= right) {
            return quickSelect(nums, start, right, k);
        }
        if (start + k - 1 >= left) {
            return quickSelect(nums, left, end, k - (left - start));
        }
        return pivot;
    }
}