## Google Interview Question for Software Engineers

• 2

Country: United States
Interview Type: In-Person

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2
of 2 vote

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SOLUTION 1

``````public Integer random(int[] weights) {
int totalWeight = 0;
Integer selected = null;
Random rand = new Random();
for(int i = 0; i < weights.length; i++) {
int r = rand.nextInt(totalWeight + weights[i]);
if(r >= totalWeight) {
selected = i;
totalWeight += weights[i];
}
}
return selected;
}``````

FOLLOW-UP

``````//main
//RandomSelectTree tree = new RandomSelectTree();
//while(tree.n > 0) tree.randomPoll();

//Segment Tree O(nlogn)
public class RandomSelectTree {

int[] tree;
int[] weights;
int n; //number of remaining balls

RandomSelectTree(int[] weights) {
tree = new int[2 * weights.length + 1];
this.weights = weights;
n = weights.length;
buildTree(0, 0, weights.length - 1);
}
//build segment tree for weights[]
private void buildTree(int root, int start, int end) {
if(start > end) return;
if(start == end) {
tree[root] = weights[start];
return;
}
int mid = (start + end) / 2;
int leftChild = 2 * root + 1, rightChild = 2 * root + 2;
buildTree(leftChild, start, mid);
buildTree(rightChild, mid + 1, end);
tree[root] = tree[leftChild] + tree[rightChild];
}

public int randomPoll() {
if(n == 0) {
//Exception
}
int rand = new Random().nextInt(tree);
int start = 0, end = weights.length - 1, node = 0;
while(start < end) {
//chosen ball is between start to (start + end) / 2
if(rand < tree[node * 2 + 1]) {
node = node * 2 + 1;
end = (start + end) / 2;
} else { //chosen ball between (start + end) / 2 + 1 and end
rand -= tree[node * 2 + 1];  //!!! narrow down to remaining range
node = node * 2 + 2;
start = (start + end) / 2 + 1;
}
}
delete(start, node);
return start;
}

//remove ball at idx.
private void delete(int idx, int node) {
n--;
while(node > 0) {
tree[node] -= weights[idx];
node = (node - 1) / 2;
}
tree -= weights[idx];
}``````

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0
of 0 vote

``````import java.util.Random;

public class RandomBallExtraction {
Random random = new Random();

class Ball {
public final int index;
public final int weight;
Ball(int index, int weight) {
this.index = index;
this.weight = weight;
}
}

int[] chooseBalls(int[] weights) {
int n = weights.length;
int weightSum = 0;
for (int i = 0; i < n; ++i) {
weightSum += weights[i];
}

Ball[] balls = new Ball[n];
for (int i = 0; i < n; ++i) {
balls[i] = new Ball(i, weights[i]);
}

int[] sequence = new int[n];
chooseBalls(sequence, 0, balls, weights.length, weightSum);

return sequence;
}

void chooseBalls(int[] sequence, int index, Ball[] balls, int n, int weightSum) {
if (n == 0) return;

StringBuilder debug = new StringBuilder();
debug.append("Debug: ");

int x = random.nextInt(weightSum);
debug.append("rand: " + x + ", ");

Ball selected = null;
for (int i = 0; i < n; ++i) {
x -= balls[i].weight;
if (x < 0) {
debug.append("chosen: " + balls[i].index + ", weight: " + balls[i].weight);
selected = balls[i];
swap(balls, i, n - 1);
break;
}
}

System.out.println(debug);

sequence[index] = selected.index;
chooseBalls(sequence, index + 1, balls, n - 1, weightSum - selected.weight);
}

void swap(Ball[] balls, int i, int j) {
Ball tmp = balls[i];
balls[i] = balls[j];
balls[j] = tmp;
}

public static void main(String[] args) {
RandomBallExtraction rbe = new RandomBallExtraction();
int[] weights = {5, 10, 35};

for (int t = 0; t < 1000; ++t) {
int[] sequence = rbe.chooseBalls(weights);

for (int i : sequence) {
System.out.println(weights[i] + " ");
}
System.out.println();
}

}
}``````

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