力扣递归 广度优先搜索之102. 二叉树的层序遍历

class Solution {
    List<List<Integer>> result = new ArrayList<>();

    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return result;
        }
        traverse(root, 0);
        return result;
    }

    private void traverse(TreeNode node, int level) {
        if (result.size() == level) {
            result.add(new ArrayList<>());
        }
        result.get(level).add(node.val);

        if (node.left != null) {
            traverse(node.left, level + 1);
        }
        if (node.right != null) {
            traverse(node.right, level + 1);
        }
    }
}

 

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        while (!stack.isEmpty()) {
            List<Integer> level = new ArrayList<>();
            Stack<TreeNode> nextLevel = new Stack<>();
            
            while (!stack.isEmpty()) {
                TreeNode node = stack.pop();
                level.add(node.val);
                
                if (node.right != null) {
                    nextLevel.push(node.right);
                }
                if (node.left != null) {
                    nextLevel.push(node.left);
                }
            }
            
            result.add(level);
            stack = nextLevel;
        }
        
        return result;
    }
}

 

 

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> level = new ArrayList<>();
            
            for (int i = 0; i < size; 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);
                }
            }
            
            result.add(level);
        }
        
        return result;
    }
}