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package Chap4;
import java.util.LinkedList;
import java.util.Queue;
/**
* 操作给定的二叉树，将其变换为源二叉树的镜像。
*/
public class MirrorTree {
private class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
/**
* 递归版本
*/
public void mirrorRecur(TreeNode root) {
exchangeChildren(root);
}
private void exchangeChildren(TreeNode node) {
if (node == null) {
return;
}
if (node.left == null && node.right == null) {
return;
}
// 交换两个子结点
TreeNode temp = node.left;
node.left = node.right;
node.right = temp;
if (node.left != null) exchangeChildren(node.left);
if (node.right != null) exchangeChildren(node.right);
}
/**
* 非递归版本,层序遍历
*/
public void mirror(TreeNode root) {
if (root == null) {
return;
}
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()) {
TreeNode node = queue.poll();
// 交换两个子结点
if (node.left != null || node.right != null) {
TreeNode temp = node.left;
node.left = node.right;
node.right = temp;
}
if (node.left != null) queue.offer(node.left);
if (node.right != null) queue.offer(node.right);
}
}
/**
* 非递归，前序遍历
*/
public void mirror_preOrder(TreeNode root) {
LinkedList<TreeNode> stack = new LinkedList<>();
// 当前结点不为空，或者为空但有可以返回的父结点（可以进行pop操作）都可以进入循环
while (root != null || !stack.isEmpty()) {
while (root != null) {
stack.push(root);
// 交换两个子结点
if (root.left != null || root.right != null) {
TreeNode temp = root.left;
root.left = root.right;
root.right = temp;
}
root = root.left;
}
if (!stack.isEmpty()) {
root = stack.pop();
root = root.right;
}
}
}
}