剑指offer学习6（Java语言版）

面试题41：数据流中的中位数

public class Demo {

	public static void main(String[] args) {
		Demo demo = new Demo();
		System.out.println(demo.getMedian()); // 两个空堆
		demo.insert(1);
		demo.insert(2);
		System.out.println(demo.getMedian()); // 偶数个
		demo.insert(3);
		System.out.println(demo.getMedian()); // 奇数个
	}

	private int count = 0;
	private PriorityQueue<Integer> minHeap = new PriorityQueue<>();
	private PriorityQueue<Integer> maxHeap = new PriorityQueue<Integer>(15,
			new Comparator<Integer>() {
				@Override
				public int compare(Integer o1, Integer o2) {
					return o2 - o1;
				}
			});

	public void insert(Integer num) {
		if ((count & 1) == 0) {
			maxHeap.offer(num);
			int filteredMaxNum = maxHeap.poll();
			minHeap.offer(filteredMaxNum);
		} else {
			minHeap.offer(num);
			int filteredMinNum = minHeap.poll();
			maxHeap.offer(filteredMinNum);
		}
		count++;
	}

	public Double getMedian() {
		if (count == 0){
			
			return 0.0;
		}
		if ((count & 1) == 0) {
			return new Double((minHeap.peek() + maxHeap.peek())) / 2;
		} else {
			return new Double(minHeap.peek());
		}
	}
}

面试题42：连续子数组的最大和

public class Demo {
	public static void main(String[] args) {
		Demo demo = new Demo();
		int[] arr = { 1, -2, 3, 10, -4, 7, 2, -5 };
		int maxSum = demo.findGreatestSumOfSubArray(arr);
		System.out.println(maxSum);
	}

	boolean invalid = false;

	public int findGreatestSumOfSubArray(int[] arr) {
		if (arr == null || arr.length <= 0) {
			invalid = true;
			return 0;
		}

		int maxSum = Integer.MIN_VALUE;
		int sum = 0;

		for (int i = 0; i < arr.length; i++) {
			sum += arr[i];
			if (sum < 0) sum = 0;
			if (maxSum < sum)
				maxSum = sum;
			// System.out.println("i = " + i + " maxSum: " + maxSum + " sum: " + sum);
		}

		return maxSum;
	}
}

面试题43：1~n整数中1出现的次数

从1到n整数中1出现的次数（Java）

面试题44：数字序列中某一位的数字

public class Demo {
	public static void main(String[] args) {
		Demo demo = new Demo();
		int digit = demo.digitAtIndex(10);
		System.out.println(digit);
	}

	public int digitAtIndex(int index) {
		if (index < 0) return -1;
		if (index < 10) return index;
		int curIndex = 10, length = 2;
		int boundNum = 10;
		while (curIndex + lengthSum(length) < index) {
			curIndex += lengthSum(length);
			boundNum *= 10;
			length++;
		}
		int addNum = (index - curIndex) / length;
		int curNum = boundNum + addNum;
		return Integer.toString(curNum).charAt(
				index - curIndex - addNum * length) - '0';
	}

	public static int lengthSum(int length) {
		int count = 9;
		for (int i = 1; i < length; i++)
			count *= 10;
		return count * length;
	}

}