Partition Array.java

M
1527814146
tags: Array, Two Pointers, Quick Sort, Sort

给一串数字, 和 int k. 根据k的值partition array, 找到第一个i, nums[i] >= k.

#### Two Pointer
- Quick sort的基础. 
- Partition Array根据pivot把array分成两半。
- 从array两边开始缩进。while loop到遍历完。非常直白的implement。
- 注意low/high,或者叫start/end不要越边界
- O(n)
- 注意: 这里第二个inner while `while(low <= high && nums[high] >= pivot) {..}` 采用了 `nums[high] >= pivot`
- 原因是题目要找第一个nums[i] >= k, 也就是说, 即便是nums[i]==k也应该swap到前面去
- 这个跟quick sort 原题有一点点不一样.


```
/*
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)?

Tags Expand 
Two Pointers Sort Array

Thoughts:
Two pointer sort, still similar to quick sort.
Move small to left and large to right.
When the two pinter meets, that's crossing point about pivot k

*/
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) {
        if (nums == null || nums.length == 0) {
            return 0;
        }
        return helper(nums, 0, nums.length - 1, k);
    }

    public void swap(int[] nums, int x, int y){
        int temp = nums[x];
        nums[x] = nums[y];
        nums[y] = temp;
    }
    
    public int helper(int[] nums, int start, int end, int pivot) {
        int low = start;
        int high = end;
        while (low <= high) {
            while(low <= high && nums[low] < pivot) {
                low++;
            }
            while(low <= high && nums[high] >= pivot) {
                high--;
            }
            if (low <= high) {
                swap(nums, low, high);
                low++;
                high--;
            }
        }
        return low;
    }
}

```