算法随笔之二叉树的遍历

class TreeNode {
      int val;
      TreeNode left;
      TreeNode right;
      TreeNode(int x) { val = x; }
 }

1、前序遍历（根-左-右）

//递归
List<Integer> list = new ArrayList<Integer>();
public List<Integer> preorderTraversal(TreeNode root) {
	if (root == null) {
		return new ArrayList();
	}
	pre(root);
	return list;
}

private void pre(TreeNode node) {
	if(node!=null){
		list.add(node.val);
		this.pre(node.left);
		this.pre(node.right);
	}
}

//迭代
public List<Integer> preOrderIteration(TreeNode root) {
	if (root == null) {
		return new ArrayList();
	}
	List<Integer> list = new ArrayList();
	Stack<TreeNode> stack = new Stack<TreeNode>();
	stack.push(root);
	while (!stack.isEmpty()) {
		TreeNode node = stack.pop();
		list.add(node.val);
		if (node.right != null) stack.push(node.right);
		if (node.left != null) stack.push(node.left);
	}
	return list;
}

2、中序遍历（左-根-右）

//递归
public List<Integer> inorderTraversal(TreeNode root) {
	if (root == null) return new ArrayList();
	preInorder(root);
	return list;
}
public void preInorder(TreeNode root){
	if (root != null){
		pre(root.left);
		list.add(root.val);
		pre(root.right);
	} 
}

//迭代
public List<Integer> inOrderIteration(TreeNode root) {
	if (root == null) return new ArrayList();
	List<Integer> list = new ArrayList<Integer>();
	Stack<TreeNode> stack = new Stack<TreeNode>();
	TreeNode curr = root;
	while (!stack.isEmpty()||curr != null) {
		while (curr != null) {
			stack.push(curr);
			curr = curr.left;
		}
		TreeNode node = stack.pop();
		list.add(node.val);
		if (node.right != null){
			curr =node.right;
		}
	}
	return list;
}

3、后续遍历（左-右-根）

//递归
List<Integer> list = new ArrayList<Integer>();
public List<Integer> postorderTraversal(TreeNode root) {
	if (root == null) return new ArrayList();
	pre(root);
	return list;
}
public void pre(TreeNode root){
	if (root != null) {
		pre(root.left);
		pre(root.right);
		list.add(root.val);
	}
}

//迭代
public List<Integer> postorderTraversal(TreeNode root) {
	if (root == null) return new ArrayList();
	List<Integer> list = new ArrayList<Integer>();
	Stack<TreeNode> stack = new Stack<TreeNode>();
	TreeNode cur = root;
	stack.push(root);
	while (!stack.isEmpty()) {
		TreeNode peek = stack.peek();
		if (peek.left != null && peek.left != cur && peek.right != cur) {
			stack.push(peek.left);
		} else if (peek.right != null && peek.right != cur) {
			stack.push(peek.right);
		} else {
			TreeNode node = stack.pop();
			list.add(node.val);
			cur = peek;
		}
	}
	return list;
}