Sort Integers

Given an integer array, sort it in ascending order. Use selection sort, bubble sort, insertion sort or any O(n2) algorithm.

Example

Example 1:
	Input:  [3, 2, 1, 4, 5]
	Output: [1, 2, 3, 4, 5]
	
	Explanation: 
	return the sorted array.

Example 2:
	Input:  [1, 1, 2, 1, 1]
	Output: [1, 1, 1, 1, 2]
	
	Explanation: 
	return the sorted array.

思路：经典排序算法：merge sort和quick sort，注意merge sort merge的时候，index起始点是start，不是0；

public class Solution {
    /**
     * @param A: an integer array
     * @return: nothing
     */
    // 1. Merge sort;
    // 2. Quick sort;
    public void sortIntegers(int[] A) {
        if(A == null || A.length == 0) {
            return;
        }
        int[] temp = new int[A.length];
        mergeSort(A, 0, A.length -1, temp);
    }
    
    private void mergeSort(int[] A, int start, int end, int[] temp) {
        if(start >= end) {
            return;
        }
        int mid = start + (end - start) / 2;
        mergeSort(A, start, mid, temp);
        mergeSort(A, mid+1, end, temp);
        merge(A, start, end, temp);
    }
    
    private void merge(int[] A, int start, int end, int[] temp) {
        int mid = start + (end - start) / 2;
        int astart = start;
        int aend = mid;
        
        int bstart = mid+1;
        int bend = end;
        
        int index = start; // 注意index = start，不是0；
        while(astart <= aend && bstart <= bend) {
            if(A[astart] < A[bstart]) {
                temp[index++] = A[astart++];
            } else {
                temp[index++] = A[bstart++];
            }
        }
        
        while(astart <= aend) {
            temp[index++] = A[astart++];
        }
        
        while(bstart <= bend) {
            temp[index++] = A[bstart++];
        }
        
        for(int i = start; i <= end; i++) {
            A[i] = temp[i];
        }
    }
}

quick sort，要熟悉两个while一个if的模板；

public class Solution {
    /**
     * @param A: an integer array
     * @return: nothing
     */
    public void sortIntegers(int[] A) {
        // quick sort;
        if(A == null || A.length == 0) {
            return;
        }
        quickSort(A, 0, A.length - 1);
    }
    
    private void quickSort(int[] A, int start, int end) {
        if(start >= end) {
            return;
        }
        int mid = start + (end - start) / 2;
        int pivot = A[mid];
        int left = start; 
        int right = end;
        
        while(left <= right) {
            while(left <= right && A[left] < pivot) {
                left++;
            }
            
            while(left <= right && A[right] > pivot) {
                right--;
            }
            
            if(left <= right) { // 这个if很关键，跟上面两个while是同样的判断；
                int temp = A[left];
                A[left] = A[right];
                A[right] = temp;
                left++;
                right--;  
            }
        }
        quickSort(A, start, right);
        quickSort(A, left, end);
    }
}