层序遍历
遍历顺序为:
F B G A D I C E H
一层一层遍历
代码
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
// 层次遍历
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> ret = new ArrayList<List<Integer>>();
if (root == null) {
return ret;
}
Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while (!queue.isEmpty()) {
List<Integer> level = new ArrayList<Integer>();
int currentLevelSize = queue.size();
for (int i = 1; i <= currentLevelSize; ++i) {
TreeNode node = queue.poll();
level.add(node.val);
if (node.left != null) {
queue.offer(node.left);
}
if (node.right != null) {
queue.offer(node.right);
}
}
ret.add(level);
}
return ret;
}
public static void main(String[] args) {
TreeNode node = new TreeNode(3);
TreeNode node3 = new TreeNode(9);
TreeNode node4 = new TreeNode(20);
TreeNode node2 = null;
TreeNode node5 = null;
TreeNode node15 = new TreeNode(15);
TreeNode node7 = new TreeNode(7);
node.left=node3;
node.right=node4;
node.right.left=node15;
node.right.right=node7;
List<List<Integer>> lists = new Solution().levelOrder(node);
System.out.println(lists);
}
}
class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode() {}
TreeNode(int val) { this.val = val; }
TreeNode(int val, TreeNode left, TreeNode right) {
this.val = val;
this.left = left;
this.right = right;
}
}