前提
1 public class TreeNode { 2 int val; 3 TreeNode left; 4 TreeNode right; 5 6 TreeNode() { 7 } 8 9 TreeNode(int val) { 10 this.val = val; 11 } 12 13 TreeNode(int val, TreeNode left, TreeNode right) { 14 this.val = val; 15 this.left = left; 16 this.right = right; 17 } 18 } 19 20 // 构造二叉树 21 TreeNode node = new TreeNode(5); 22 node.right = new TreeNode(3); 23 TreeNode left = node.left = new TreeNode(1); 24 left.left = new TreeNode(2); 25 left.right = new TreeNode(4);
前序
1 private static List<Integer> preorderTraversal(TreeNode head) { 2 List<Integer> res = new ArrayList<>(); 3 if (head == null) { 4 return res; 5 } 6 7 Stack<TreeNode> stack = new Stack<>(); 8 stack.push(head); 9 while (!stack.isEmpty()) { 10 TreeNode top = stack.pop(); 11 res.add(top.val); 12 if (top.right != null) { 13 stack.push(top.right); 14 } 15 if (top.left != null) { 16 stack.push(top.left); 17 } 18 } 19 System.out.println(res); 20 return res; 21 }
后序
private static List<Integer> postorderTraversal(TreeNode root) { List<Integer> res = new LinkedList<>(); if (root == null) { return res; } Stack<TreeNode> stack = new Stack<>(); stack.push(root); while (!stack.isEmpty()) { TreeNode top = stack.pop(); res.add(0, top.val); if (top.left != null) { stack.push(top.left); } if (top.right != null) { stack.push(top.right); } } System.out.println(res); return res; }
中序
private static List<Integer> inorderTraversal(TreeNode root) { List<Integer> res = new ArrayList<>(); if (root == null) { return res; } Stack<TreeNode> stack = new Stack<>(); while (root != null || !stack.isEmpty()) { if (root != null) { stack.push(root); root = root.left; continue; } TreeNode top = stack.pop(); res.add(top.val); root = top.right; } System.out.println(res); return res; }
附录
递归代码
1 public static List<Integer> res = new ArrayList<>(); 2 private static List<Integer> preorderTraversalRecur(TreeNode head) { 3 if (head == null) { 4 return res; 5 } 6 // 这里放其他位置,就是前序/中序/后序遍历 7 res.add(head.val); 8 preorderTraversalRecur(head.left); 9 preorderTraversalRecur(head.right); 10 return res; 11 }