144.二叉树的前序遍历
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
if(root==null) return res;
Stack<TreeNode> stack = new Stack<>();
stack.add(root);
while(!stack.isEmpty()){
TreeNode node = stack.pop();
res.add(node.val);
if(node.right!=null){
stack.push(node.right);
}
if(node.left!=null){
stack.push(node.left);
}
}
return res;
}
}
102.二叉树的层次遍历
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
if(root==null) return res;
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
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.add(node.left);
}
if(node.right!=null){
queue.add(node.right);
}
}
res.add(list);
}
return res;
}
}
