算法-队列

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树的层序遍历问题

从上到下

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package leetcode.queue;
import leetcode.stack.TreeNode;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
 * Created by cdx0312
 * 2018/4/8
 */
public class BinaryTreeLevelOrderTraversal_102 {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null)
            return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null)
                    queue.offer(node.left);
                if (node.right != null)
                    queue.offer(node.right);
            }
            res.add(list);
        }
        return res;
    }
}

从下到上

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package leetcode.queue;
import leetcode.stack.TreeNode;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
 * Created by cdx0312
 * 2018/4/8
 */
public class BinaryTreeLevelOrderTraversalII_107 {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null)
            return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null)
                    queue.offer(node.left);
                if (node.right != null)
                    queue.offer(node.right);
            }
            res.add(0, list);
        }
        return res;
    }
}

之字形层序遍历

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package leetcode.queue;
import leetcode.stack.TreeNode;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
 * Created by cdx0312
 * 2018/4/8
 */
public class BinaryTreeZigzagLevelOrderTraversal_103 {
    public List<List<Integer>> zigzagLevelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null)
            return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int flag = 1;
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (flag % 2 == 1)
                    list.add(node.val);
                else
                    list.add(0, node.val);
                if (node.left != null)
                    queue.offer(node.left);
                if (node.right != null)
                    queue.offer(node.right);
            }
            res.add(list);
            flag++;
        }
        return res;
    }
}

从一颗树右侧看过去的节点数

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package leetcode.queue;
import leetcode.stack.TreeNode;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
 * Created by cdx0312
 * 2018/4/8
 */
public class BinaryTreeRightSideView_199 {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null)
            return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (i == size - 1)
                    res.add(node.val);
                if (node.left != null)
                    queue.offer(node.left);
                if (node.right != null)
                    queue.offer(node.right);
            }
        }
        return res;
    }
}

合并k个排序链表, 用优先队列，重写compare方法

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package leetcode.queue;
import leetcode.linkedList.ListNode;
import java.util.Comparator;
import java.util.List;
import java.util.PriorityQueue;
/**
 * Created by cdx0312
 * 2018/4/8
 */
public class MergeKSortedLists_23 {
    public ListNode mergeKLists(ListNode[] lists) {
        if (lists == null || lists.length == 0)
            return null;
        PriorityQueue<ListNode> queue = new PriorityQueue<>(lists.length, new Comparator<ListNode>() {
            @Override
            public int compare(ListNode o1, ListNode o2) {
                return o1.val - o2.val;
            }
        });
        ListNode dummy = new ListNode(0);
        ListNode p = dummy;
        for (ListNode node : lists) {
            if (node != null)
                queue.add(node);
        }
        while (!queue.isEmpty()) {
            p.next = queue.poll();
            p = p.next;
            if (p.next != null)
                queue.add(p.next);
        }
        return dummy.next;
    }
}

找到数组中出现频率最高的k个数

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package leetcode.queue;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.PriorityQueue;
/**
 * Created by cdx0312
 * 2018/4/8
 */
public class TopKFrequentElements_347 {
    public List<Integer> topKFrequent(int[] nums, int k) {
        List<Integer> res = new ArrayList<>();
        //统计数组中的元素出现的频率，key-num, value-次数
        HashMap<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(nums[i])) {
                map.put(nums[i], map.get(nums[i])+1);
            } else {
                map.put(nums[i],1);
            }
        }
        //扫描map，维护当前频率最高的k个元素,
        PriorityQueue<Pair> queue = new PriorityQueue<>(k);
        for (Integer i : map.keySet()) {
            if (queue.size() == k) {
                if (map.get(i) > queue.peek().freq){
                    queue.poll();
                    queue.add(new Pair(map.get(i),i));
                }
            } else {
                queue.add(new Pair(map.get(i), i));
            }
        }
        //遍历优先队列，将num加入到结果集中
        for (Pair pair : queue) {
            res.add(0, pair.num);
        }
        return res;
    }
    class Pair implements Comparable<Pair>{
        int freq;
        int num;
        public Pair(int freq, int num) {
            this.freq = freq;
            this.num = num;
        }
        @Override
        public int compareTo(Pair o) {
            return this.freq - o.freq;
        }
    }
}
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