深度优先搜索和广度优先搜索

/**
 * BFS:利用队列
 */
public static List<Integer> bfs(TreeNode root) {
	Queue<TreeNode> treeNodeQueue = new LinkedList<>();
	List<Integer> integerList = new ArrayList<>();
	if (root == null) return new ArrayList<>();
	((LinkedList<TreeNode>) treeNodeQueue).add(root);
	while (!treeNodeQueue.isEmpty()) {
		TreeNode treeNode = treeNodeQueue.poll();
		integerList.add(treeNode.val);
		if (treeNode.left != null) {
			((LinkedList<TreeNode>) treeNodeQueue).add(treeNode.left);
		}
		if (treeNode.right != null) {
			((LinkedList<TreeNode>) treeNodeQueue).add(treeNode.right);
		}
	}
	return integerList;
}

/**
 * DFS:先序遍历，非递归方式利用 栈
 */
public static List<Integer> dfs(TreeNode root) {
	Stack<TreeNode> treeNodeStack = new Stack<>();
	List<Integer> integerList = new ArrayList<>();
	if (root == null) return new ArrayList<>();
	treeNodeStack.add(root);
	while (!treeNodeStack.isEmpty()) {
		TreeNode treeNode = treeNodeStack.pop();
		integerList.add(treeNode.val);
		if (treeNode.right != null) treeNodeStack.add(treeNode.right);
		if (treeNode.left != null) treeNodeStack.add(treeNode.left);
	}
	return integerList;
}