Binary Tree Level Order Traversal

Level Order Of Binary Tree

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

Input : 3 / \ 4 5 /\ 6 7 Output : [[3], [4,5],[6,7]]

์ž๋ฃŒ๊ตฌ์กฐ : Queue(FIFO) - BFS(๋„ˆ๋น„์šฐ์„ ), Stack(FILO) - DFS(๊นŠ์ด์šฐ์„ )

์•Œ๊ณ ๋ฆฌ์ฆ˜ 0. ํ์˜ ์‚ฌ์ด์ฆˆ ์ฒดํฌ!, ๋ฆฌ์ŠคํŠธ ์ƒ์„ฑ! - level๋ณ„๋กœ ๋บ„๊ฒƒ์ด๊ธฐ ๋•Œ๋ฌธ์— level ๋ณ„๋กœ ๋ฆฌ์ŠคํŠธ ์ƒ์„ฑํ•œ๋‹ค.(ํ์—์„œ ๋นผ๋‚ด๋Š” ๋ฐ˜๋ณต๋ฌธ ๋“ค์–ด๊ฐ€๊ธฐ ์ „์— ๋ฆฌ์ŠคํŠธ๋ฅผ ์ƒ์„ฑํ•œ๋‹ค.) 1. ํ์— ๋…ธ๋“œ ๋„ฃ๋Š”๋‹ค. 2. ๋…ธ๋“œ ๋นผ๋ฉด์„œ ์™ผ์ชฝ๊ณผ ์˜ค๋ฅธ์ชฝ์— ๋…ธ๋“œ๊ฐ€ ์žˆ๋Š”์ง€ ํ™•์ธํ•œ๋‹ค. - ์™ผ/์˜ค ์ˆœ์„œ๋Š” ๋ฌธ์ œ์—์„œ ์ œ์‹œ. ๋…ธ๋“œ ๋นผ๋ฉด์„œ ๋ฌธ์ œ์—์„œ ์ œ์‹œ๋œ ์ˆœ์„œ(์—ฌ๊ธฐ์„œ๋Š” ์™ผ->์˜ค)๋กœ ๋…ธ๋“œ ๋‹ค์‹œ ๋„ฃ๋Š”๋‹ค.

์•Œ๊ณ ๋ฆฌ์ฆ˜์„ Java๋กœ ๊ตฌํ˜„

/**
 * Definition for a binary tree node.
 * public 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;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        //left->right : BFS, BFS = Queue!
        List<List<Integer>> result = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        //ํ๊ฐ€ ๋นŒ ๋•Œ๊นŒ์ง€ ํ์— ๋…ธ๋“œ ๋„ฃ๊ณ  ๋นผ๋Š” ๊ฒƒ์„ ๋ฐ˜๋ณตํ•œ๋‹ค.
        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);}
            }
            result.add(list);
        }
        return result;
    }
}

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