Given a 2D integer array matrix, return the transpose of matrix.

The transpose of a matrix is the matrix flipped over its main diagonal, switching the matrix’s row and column indices.

Example 1:

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[1,4,7],[2,5,8],[3,6,9]]

Example 2:

Input: matrix = [[1,2,3],[4,5,6]]
Output: [[1,4],[2,5],[3,6]]

class Solution {
public…

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one.

You must implement a solution with a linear runtime complexity and use only constant extra space.

Example 1:

Input: nums = [2,2,1]
Output: 1

Example 2:

Input: nums = [4,1,2,1,2]
Output: 4

Example 3:

Input: nums = [1]
Output: 1

constant extra space

題目限制不會變動的空間，例如array，會變動的空間像是vector的push_back就不行。

解法一

class Solution {
public:
    int singleNumber(vector<int>& nums) {
        
        int arr_pos[3*10000+1] = {0};
        int arr_neg[3*10000] = {0};
        
        for(int i=0; i<nums.size(); i++){
            if(nums[i] >= 0){
                arr_pos[nums[i]]++;
            }
            else{
                arr_neg[-(nums[i])]++;
            }
        }
       
        for(int i=0; i<=3*10000; i++){
            if(arr_pos[i] == 1){
                return i;
            }
            
            if(arr_neg[i] == 1){
                    return -i;
            }
        }
        return 0;
    }
};

解法二

使用XOR解

以example 2為例:

Input: nums = [4,1,2,1,2]
Output: 4

由左至右，計算如下，有黃色標示為輸入數字，未標示者為res

Spinner 下拉式選單

1. 在strings.xml中建立字串陣列，再將此字串陣列設定給spinner元件成為選單

<resources>
    <string name="app_name">spinner</string>
    <string-array name="gender_list">
        <item>男生</item>
        <item>女生</item>
    </string-array>
</resources>

2. 在介面布局檔(activity_main.xml)中加入spinner元件，並設定好屬性

<Spinner
    android:id="@+id/spnGender"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginLeft="20dp"
    android:layout_marginTop="20dp"
    android:entries="@array/gender_list" />

3. MainActivity.java中取得spiinner元件，並設定處理程序

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Spinner mspnGender = findViewById(R.id.spnGender);
    mspnGender.setOnItemSelectedListener(new Spinner.OnItemSelectedListener() {        @Override
        public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
            //選擇選項要做的事
            switch (position){
                case 0:
                    mGender = "男生";
                    break;
                case 1:
                    mGender = "";
                    break;
            }
        }

        @Override
        public void onNothingSelected(AdapterView<?> parent) {
            //沒有選擇選項要做的事
        }
    });

在xml的layout_margin屬性中，有幾個非常相似的地方

android:layout_marginStart / android:layout_marginLeft

android:layout_marginEnd / android:layout_marginRight

我試了一下，兩個用起來沒有差別。

來看一下android developers的doc.是怎麼寫的