題目來源：https://leetcode.com/contest/weekly-contest-106/problems/sort-array-by-parity-ii/

問題描述

922. Sort Array By Parity II

Given an array A of non-negative integers, half of the integers in A are odd, and half of the integers are even.

Sort the array so that whenever A[i] is odd, 

You may return any answer array that satisfies this condition.

Example 1:

Input: [4,2,5,7]

Output: [4,5,2,7]

Explanation: [4,7,2,5], [2,5,4,7], [2,7,4,5] would also have been accepted.

Note:

1. 2 <= A.length <= 20000
2. A.length % 2 == 0
3. 0 <= A[i] <= 1000

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題意

給定一個長度為n（n是偶數）的數列，有n/2個元素是偶數，n/2個元素是奇數。求數列的一個重排，使得偶數下標的元素都是偶數，奇數下標的元素都是奇數。

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思路

設定兩個指標i和j分別遍歷數列的奇數下標和偶數下標，如果發現i下標位置和j下標位置下標與元素的奇偶性不同就交換。

以後千萬要注意，“==”“!=”的優先順序比位運算高！！！！！！！！

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程式碼

class Solution {
public:
    vector<int> sortArrayByParityII(vector<int>& A) {
        int i= 0 , j = 1, n = A.size();
        while (i<n && j<n)
        {
            while ((A[i] & 1) == 0)
            {
                i += 2;
            }
            if (i >= n)
            {
                break;
            }
            while ((A[j] & 1) == 1)
            {
                j += 2;
            }
            if (j >= n)
            {
                break;
            }
            swap(A[i], A[j]);
        }
        return A;
    }
};