二叉树学习——前序、中序、后序遍历（Java实现）

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一些学习总结

递归实现:

1.前序遍历

public static ArrayList<Integer> preorderTraversal(TreeNode root) {ArrayList<Integer> sort = new ArrayList<>();if (root == null) {return sort;}if (root.left == null && root.right == null) {sort.add(root.val);return sort;}sort.add(root.val);sort.addAll(preorderTraversal(root.left));sort.addAll(preorderTraversal(root.right));return sort;}

2.中序遍历

public static ArrayList<Integer> inorderTraversal(TreeNode root) {ArrayList<Integer> sort = new ArrayList<>();if (root == null) {return sort;}if (root.left == null && root.right == null) {sort.add(root.val);return sort;}sort.addAll(inorderTraversal(root.left));sort.add(root.val);sort.addAll(inorderTraversal(root.right));return sort;}

3.后序遍历

public static ArrayList<Integer> postorderTraversal(TreeNode root) {ArrayList<Integer> sort = new ArrayList<>();if (root == null) {return sort;}if (root.left == null && root.right == null) {sort.add(root.val);return sort;}sort.addAll(postorderTraversal(root.left));sort.addAll(postorderTraversal(root.right));sort.add(root.val);return sort;}

非递归实现

4. 前序遍历

public static ArrayList<Integer> preorderTraversal(TreeNode root) {ArrayList<Integer> sort = new ArrayList<>();TreeNode cur = root;Stack<TreeNode> stack = new Stack<>();while (cur != null || !stack.isEmpty()) {while (cur != null) {sort.add(cur.val);stack.push(cur);cur = cur.left;}cur = stack.pop();cur = cur.right;}return sort;}

5.中序遍历

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

6.后序遍历

public static ArrayList<Integer> postorderTraversal(TreeNode root) {ArrayList<Integer> sort = new ArrayList<>();Stack<TreeNode> stack = new Stack<>();Stack<TreeNode> stack2 = new Stack<>();TreeNode cur = root;while (cur != null || !stack.isEmpty()) {while (cur != null) {stack.push(cur);stack2.push(cur);cur = cur.right;}cur = stack.pop();cur = cur.left;}sort.add(stack2.pop().val);while (stack2.size() > 0) {cur = stack2.pop();sort.add(cur.val);}return sort;}

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