107. Binary Tree Level Order Traversal II

leetcode.com/problems/binary-tree-level-order-traversal-ii/submissions/

Binary Tree Level Order Traversal II - LeetCode

/**
 * 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>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<>();
        if(root == null) return ret;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        
        while(!q.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            int size = q.size();
            
            for(int i=0; i<size; i++) {
                TreeNode node = q.poll();
                level.add(node.val);
                if(node.left != null) q.offer(node.left);
                if(node.right != null) q.offer(node.right);
            }
            ret.add(0, level);
        }
        
        return ret;
    }
}

q.offer() : insert

q.poll() : pop

q.peek() : getHead

coding-factory.tistory.com/603?category=758267

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