Partition Array

lintcode 31

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
Example 1:

Input:
[],9
Output:
0

Example 2:

Input:
[3,2,2,1],2
Output:1
Explanation:
the real array is[1,2,2,3].So return 1
Challenge
Can you partition the array in-place and in O(n)?

Notice
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

这一个解法不涉及end ，只有一个start pointer，当当前值小于k，改值与start 的值交换，然后start++；

public class Solution {
    /**
     * @param nums: The integer array you should partition
     * @param k: An integer
     * @return: The index after partition
     */
    public int partitionArray(int[] nums, int k) {
        // write your code here
        if (nums == null || nums.length == 0) return 0;
        int start = 0;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] < k) {
                int temp = nums[start];
                nums[start] = nums[i];
                nums[i] = temp;
                start++;
            }
        }
        return start;
    }
}

public class Solution {
    /**
     * @param nums: The integer array you should partition
     * @param k: An integer
     * @return: The index after partition
     */
    public int partitionArray(int[] nums, int k) {
        int start = 0;
        int end = nums.length - 1;
        while (start <= end) {
           while (start <= end && nums[start] < k) {
               start++;
           }
           while (start <= end && nums[end] >= k) {
               end--;
           }
           if (start <= end) {
               int temp = nums[start];
               nums[start] = nums[end];
               nums[end] = temp;
               start++;
               end--;
           }
        }
        return start;
    }
}

another method

    public int partitionArray(int[] nums, int k) {
        int start = 0;
        int end = nums.length - 1;
        while (start <= end) {
            if (nums[start] < k) {
                start++;
            } else {
                int temp = nums[end];
                nums[end] = nums[start];
                nums[start] = temp;
                end--;
            }
        }
        return start;
    }