# Partition Array

Medium

## 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.

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

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

## Thinking

• The array is not ordered. At the end, it still not ordered but all emelents which is less than k are moved to the left.
• Inspace exchange
• Have a value ‘pos’ to save the first element which is greater or equals k.
• Compare adjacent two elements. (pre and cur)
• pre < k and cur < k; pos = cur + 1 // Maybe next one is greater or equals k.
• pre < k and cur >=k; pos = cur
• pre >=k and cur < k; switch pos and cur, pos ++; // Note: at this time, pos may less than pre
• pre >=k and cur >=K; no action needed.

## Review

• First solution is ok to find the position but the array of result is not partition.
• Using two pointers to solve problem ## Solution

#### Java (Review)

``````public class Solution {
/**
*@param nums: The integer array you should partition
*@param k: As description
*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 pos = 0;
for (int i = 0; i < nums.length; i++) {
if (nums[i] < k && nums[pos] >= k) {
int tmp = nums[pos];
nums[pos] = nums[i];
nums[i] = tmp;
++pos;
} else if (nums[i] < k && nums[pos] < k) {
pos = i + 1;
} else if (nums[i] >= k && nums[pos] < k) {
++pos;
}
}
return pos;
}
}
``````

#### Java (Array of result is not correct)

``````public class Solution {
/**
*@param nums: The integer array you should partition
*@param k: As description
*return: The index after partition
*/
public int partitionArray(int[] nums, int k) {
//write your code here
if (nums == null || nums.length == 0) {
return 0;
}
if (nums.length == 1) {
if (nums < k) {
return 1;
} else {
return 0;
}
}
int tmp = nums;
int pos = 0;
if (tmp < k) {
pos = 1;
}
int index = 1;
while (index < nums.length) {
if (nums[index - 1] < k && nums[index] < k) {
pos = index + 1;
} else if (nums[index - 1] < k && nums[index] >= k) {
pos = index;
} else if (nums[index - 1] >= k && nums[index] < k) {
tmp = nums[pos];
nums[pos] = nums[index];
nums[index] = tmp;
++pos;
} else if (nums[index - 1] >= k && nums[index] >= k) {
// noop
}
++index;
}
return pos;
}
}
``````