Partition Array

Question

Given an array nums of integers and an int k, partition the array (i.e move the elements in "nums") such that:

• All elements < k are moved to the left
• All elements >= k are moved to the right

Return the partitioning index, i.e the first index i nums[i] >= k.

Example If nums = [3,2,2,1] and k=2, a valid answer is 1.

Note You should do really partition in array nums instead of just counting the numbers of integers smaller than k.

If all elements in nums are smaller than k, then return nums.length

Challenge Can you partition the array in-place and in O(n)?

Analysis

Test Cases:

• empty string
• all elements are smaller than k
• all elements are larger than k

Thoughts

和sort color的题差不多，类似quick sort的做法 这里我直接用了sort color的模板改了一下

Solution

class Solution:
    """
    @param nums: The integer array you should partition
    @param k: As description
    @return: The index after partition
    """
    def partitionArray(self, nums, k):
        # write your code here
        # you should partition the nums by k
        # and return the partition index as description
        numRed = 0 
        numBlue = 0
        length = len(nums)
        index = 0
        while index <= length - 1 - numBlue:
            if nums[index] < k:
                nums = self.swap(nums, numRed, index)
                numRed += 1
                index += 1
            elif nums[index] >= k:
                nums = self.swap(nums, length - 1 - numBlue, index)
                numBlue += 1
        return index

    def swap(self, nums, i, j):
        tmp = nums[j]
        nums[j] = nums[i]
        nums[i] = tmp
        return nums