leetcode解题: Find Largest Value in Each Tree Row (515)

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You need to find the largest value in each row of a binary tree.

Example:

1
2
3
4
5
6
7
8
9
Input: 
          1
         / \
        3   2
       / \   \  
      5   3   9 
Output: [1, 3, 9]

解法1：BFS

因为是按行来选择最大值，自然想到用BFS遍历每一行，然后存储下每一行的最大值。主要考察了BFS的写法。
Java

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> largestValues(TreeNode root) {
		// bfs
		List<Integer> result = new ArrayList<Integer>();
		if (root == null) {
			return result;
		}
		
		Queue<TreeNode> queue = new LinkedList<TreeNode>();
		queue.offer(root);
		while (!queue.isEmpty()) {
			int size = queue.size();
			int levelmax = Integer.MIN_VALUE;
			for (int i = 0; i < size; ++i) {
				TreeNode cur = queue.poll();
				levelmax = Math.max(levelmax, cur.val);
				if (cur.left != null) {
					queue.offer(cur.left);
				}
				if (cur.right != null) {
					queue.offer(cur.right);
				}
			}
			result.add(levelmax);
		}
		return result;
    }
}