Sumary of traversal of binary tree. 二叉树遍历总结

Traversal of binary tree is very import when sloving the binary tree problem.
Today I will try starting to record the traversal way I met in leetcode or somewhere else. Continuing update...

1. LevelOrder Traversal using Queue.
This is a very common traversal case. We use it to record the width of the current level.
Plus: Actually is a bfs search in a graph when you replace the left and right node to adjs of a vertex.

public class Example {

    public List<Integer> levelOrderTraversal(TreeNode root) {
        if (root == null) return null;
        List<Integer> result = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode tmp = queue.poll();
                System.out.println(tmp.val);
                result.add(tmp.val);
                if (tmp.left != null) queue.offer(tmp.left);
                if (tmp.right != null) queue.offer(tmp.right);
            }
        }
        return result;
    }

    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
    }
}