# Random Number

The code to use the nextInt() method to generate an integer within a range is:

```java
public static int generateRandomIntIntRange(int min, int max) {
    Random r = new Random();
    return r.nextInt((max - min) + 1) + min;
}
```

## Shuffle

Shuffle cards

```java
public void shuffle(int[] cards) {
    Random r = new Random();
    int len = cards.length;
    //swap random card from card[0,...51] with card[51], then 
    //swap random card from card[0,...50] with card[50], then 
    for(int i=len; i>=1; i--) {
        int rand = r.nextInt(52); //generate random num in [0,...,51]
        swap(card,rand,i);
    }
}
```

Randomly select k number from an array

```java
```

## Reservoir Sampling

```java
// call this function to add new sample and return a random sample
// 0, 1, 2, 3, 4, 5
int totalSample = 0;
int result = 0;
public int ReservoirSampling(int sample) {
    totalSample ++;
    int r = random(1,totalSample);//random int from [1,totalSample]
    if(r==1) result = sample;
    return result;
}
```

Extend: Randomly select k numbers

```java
int totalSample = 0;
int result[] = new int[k];
"Initialize all the k samples for the first k input"
public int[] ReservoirSampling(int sample) {
    totalSample ++;
    int r = random(1,totalSample);//random int from [1,totalSample]
    if(r<=k) result[r-1] = sample;
    return result;
}
```


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