二叉树遍历的非递归写法Java

先序遍历

从根节点来看，遍历顺序是根 -> 左 -> 右

我们用一个栈来储存二叉树中的节点值，访问一个节点时，先打印这个节点，再将这个节点的左子树和右子树放进栈中，先拿出左子树即可

public static void preOrderUnRecur(Node head) {
	System.out.print("pre-order: ");
	if (head != null) {
		Stack<Node> stack = new Stack<Node>();
		stack.add(head);
		while (!stack.isEmpty()) {
			head = stack.pop();
			System.out.print(head.value + " ");
			if (head.right != null) {
				stack.push(head.right);
			}
			if (head.left != null) {
				stack.push(head.left);
			}
		}
	}
	System.out.println();
}

中序遍历

从根节点来看，遍历顺序是左 -> 根 -> 右

也就是说，我们要先遍历这个二叉树的所有左子树，放进栈中，再遍历右子树

首先先将二叉树所有的左子树放进栈，再依次提出打印，而pop的条件就是当前节点的左子树或右子树为空

public static void inOrderUnRecur(Node head) {
	System.out.print("in-order: ");
	if (head != null) {
		Stack<Node> stack = new Stack<Node>();
		while (!stack.isEmpty() || head != null) {
			if (head != null) {
				stack.push(head);
				head = head.left;
			} else {
				head = stack.pop();
				System.out.print(head.value + " ");
				head = head.right;
			}
		}
	}
	System.out.println();
}

后序遍历

从根节点来看，遍历顺序是左 -> 右 -> 根

类似于根 -> 右 -> 左的逆序

public static void posOrderUnRecur1(Node head) {
	System.out.print("pos-order: ");
	if (head != null) {
		Stack<Node> s1 = new Stack<Node>();
		Stack<Node> s2 = new Stack<Node>();
		s1.push(head);
		while (!s1.isEmpty()) {
			head = s1.pop();
			s2.push(head);
			if (head.left != null) {
				s1.push(head.left);
			}
			if (head.right != null) {
				s1.push(head.right);
			}
		}
 	        while (!s2.isEmpty()) {
			System.out.print(s2.pop().value + " ");
		}
	}
	System.out.println();
}