leetcode144 二叉树的前序遍历

递归版本

public List<Integer> preorderTraversal(TreeNode root) {List<Integer> res = new ArrayList<>();preorder(root, res);return res;
}public void preorder(TreeNode root, List<Integer> res) {if (root == null) {return;}res.add(root.val);preorder(root.left, res);preorder(root.right, res);
}

非递归版本:利用栈模拟

public List<Integer> preorderTraversal(TreeNode root) {List<Integer> res = new ArrayList<>();//利用栈模拟非递归遍历Stack<TreeNode> stack = new Stack<>();if (root == null) {return res;}stack.push(root);while (!stack.isEmpty()) {TreeNode pop = stack.pop();res.add(pop.val);if (pop.right != null) {stack.push(pop.right);}if (pop.left != null) {stack.push(pop.left);}}return res;
}

leetcode94 二叉树的中序遍历

 List<Integer> list = new ArrayList<>();public List<Integer> inorderTraversal(TreeNode root) {mid(root);return list;
}private void mid(TreeNode root) {if (root == null) return;mid(root.left);list.add(root.val);mid(root.right);
}

非递归版本

public List<Integer> inorderTraversal(TreeNode root) {List<Integer> result = new ArrayList<>();if (root == null) {return result;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while (cur != null || !stack.isEmpty()) {if (cur != null) {stack.push(cur);cur = cur.left;} else {cur = stack.pop();result.add(cur.val);cur = cur.right;}}return result;
}

leetcode145 二叉树的后序遍历

public List<Integer> postorderTraversal(TreeNode root) {List<Integer> list = new ArrayList();if (root == null) {return list;}postorder(root, list);return list;
}private void postorder(TreeNode root, List<Integer> list) {if (root == null) {return;}postorder(root.left, list);postorder(root.right, list);list.add(root.val);
}

非递归版本

public List<Integer> postorderTraversal(TreeNode root) {List<Integer> res = new ArrayList<>();if (root == null) {return res;}Stack<TreeNode> stack = new Stack<>();stack.push(root);while (!stack.isEmpty()) {TreeNode node = stack.pop();res.add(node.val);if (node.left != null) {stack.push(node.left);}if (node.right != null) {stack.push(node.right);}}Collections.reverse(res);return res;
}

leetcode102 二叉树的层序遍历

public List<List<Integer>> levelOrder(TreeNode root) {List<List<Integer>> res = new ArrayList<>();Queue<TreeNode> queue = new LinkedList<>();if (root == null) {return res;}queue.add(root);while (!queue.isEmpty()) {int size = queue.size();List<Integer> temp = new ArrayList<>();for (int i = 0; i < size; i++) {TreeNode node = queue.poll();temp.add(node.val);if (node.left != null) {queue.offer(node.left);}if (node.right != null) {queue.offer(node.right);}}res.add(temp);}return res;
}

leetcode199 二叉树的右视图

在层序遍历中只加入该层的最后一个节点

public List<Integer> rightSideView(TreeNode root) {List<Integer> res = new ArrayList<>();Queue<TreeNode> queue = new LinkedList<>();if (root == null) {return res;}queue.add(root);while (!queue.isEmpty()) {int size = queue.size();for (int i = 0; i < size; i++) {TreeNode poll = queue.poll();if (poll.left != null) {queue.add(poll.left);}if (poll.right != null) {queue.add(poll.right);}if (i == size - 1) {res.add(poll.val);}}}return res;
}

leetcode637 二叉树层的平均值

public List<Double> averageOfLevels(TreeNode root) {List<Double> res = new ArrayList<>();Queue<TreeNode> queue = new LinkedList<>();if (root == null) {return res;}queue.add(root);while (!queue.isEmpty()) {double temp = 0;int size = queue.size();for (int i = 0; i < size; i++) {TreeNode node = queue.poll();temp += node.val;if (node.left != null) {queue.add(node.left);}if (node.right != null) {queue.add(node.right);}}temp = temp / size;res.add(temp);}return res;
}

leetcode429 N叉树的层序遍历

public List<List<Integer>> levelOrder(Node root) {List<List<Integer>> res = new ArrayList<>();Queue<Node> queue = new LinkedList<>();if (root == null) {return res;}queue.add(root);while (!queue.isEmpty()) {int size = queue.size();List<Integer> temp = new ArrayList<>();for (int i = 0; i < size; i++) {Node poll = queue.poll();temp.add(poll.val);List<Node> children = poll.children;for (int j = 0; j < children.size(); j++) {if (children.get(j) != null) {queue.add(children.get(j));}}}res.add(temp);}return res;
}

leetcode515 在每个树行中的最大值

public List<Integer> largestValues(TreeNode root) {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();int temp = Integer.MIN_VALUE;for (int i = 0; i < size; i++) {TreeNode poll = queue.poll();temp = Math.max(temp, poll.val);if (poll.left != null) {queue.add(poll.left);}if (poll.right != null) {queue.add(poll.right);}}res.add(temp);}return res;
}

leetcode116 填充每个节点的右侧指针

public Node connect(Node root) {Queue<Node> queue = new LinkedList<>();if (root == null) {return null;}queue.add(root);while (!queue.isEmpty()) {int size = queue.size();Node poll = queue.poll();if (poll.left != null) {queue.add(poll.left);}if (poll.right != null) {queue.add(poll.right);}for (int i = 1; i < size; i++) {Node cur = queue.poll();if (cur.left != null) {queue.add(cur.left);}if (cur.right != null) {queue.add(cur.right);}poll.next = cur;poll = cur;}}return root;
}

leetcode104 二叉树的最大深度

 public int maxDepth(TreeNode root) {if (root == null) {return 0;}int left = maxDepth(root.left);int right = maxDepth(root.right);return Math.max(left, right) + 1;
}

leetcode226 翻转二叉树

public TreeNode invertTree(TreeNode root) {if (root == null) {return root;}TreeNode t = root.left;root.left = root.right;root.right = t;invertTree(root.left);invertTree(root.right);return root;
}

leetcode101 对称二叉树

public boolean isSymmetric(TreeNode root) {if (root == null) {return true;}return check(root.left, root.right);
}public boolean check(TreeNode left, TreeNode right) {if (left == null && right != null) {return false;} else if (left != null && right == null) {return false;} else if (left == null && right == null) {return true;} else if (left.val != right.val) {return false;}//比较外侧节点boolean bool1 = check(left.left, right.right);//比较内侧节点boolean bool2 = check(left.right, right.left);return bool1 && bool2;
}