二叉树遍历
1. 先序遍历
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
preorderTraversal(root, res);
return res;
}
private void preorderTraversal(TreeNode root, List<Integer> list) {
if (root == null) return ;
list.add(root.val);
preorderTraversal(root.left, list);
preorderTraversal(root.right, list);
}public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
while (root != null || !stack.isEmpty()) {
while (root != null) {
res.add(root.val);
stack.push(root);
root = root.left;
}
root = stack.pop();
root = root.right;
}
return res;
}2. 中序遍历
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
inorderTraversal(root, res);
return res;
}
private void inorderTraversal(TreeNode root, List<Integer> list) {
if (root == null) return ;
inorderTraversal(root.left, list);
list.add(root.val);
inorderTraversal(root.right, list);
}public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
while (root != null || !stack.isEmpty()) {
while (root != null) {
stack.push(root);
root = root.left;
}
root = stack.pop();
res.add(root.val);
root = root.right;
}
return res;
}3. 后序遍历
public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
postorderTraversal(root,res);
return res;
}
private void postorderTraversal(TreeNode root, List<Integer> list) {
if (root == null) return ;
postorderTraversal(root.left, list);
postorderTraversal(root.right, list);
list.add(root.val);
}public List<Integer> postorderTraversal(TreeNode root) {
List<Integer> res = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
while (root != null || !stack.isEmpty()) {
while (root != null) {
stack.push(root);
root = root.left == null ? root.right : root.left;
}
TreeNode temp = stack.pop();
res.add(temp.val);
if (!stack.isEmpty() && stack.peek().left == temp) {
root = stack.peek().right;
}
}
return res;
}4. 层序遍历
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> resultList = new ArrayList<>();
if (root == null) return resultList;
Queue<TreeNode> queue = new LinkedList<>();
queue.add(root);
while (!queue.isEmpty()) {
List<Integer> list = new ArrayList<>();
int size = queue.size();
for (int i = 0; i < size; i++) {
if (queue.peek().left != null) queue.add(queue.peek().left);
if (queue.peek().right != null) queue.add(queue.peek().right);
list.add(queue.poll().val);
}
resultList.add(list);
}
return resultList;
}最后更新于
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