Symmetric Tree

最新推荐文章于 2019-01-05 12:57:18 发布

原创最新推荐文章于 2019-01-05 12:57:18 发布 · 4k 阅读

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二叉树 (binary tree) 专栏收录该内容

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本文介绍如何通过递归和迭代方法判断给定的二叉树是否为镜像对称结构，包括代码实现和非递归解决方案。

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

But the following is not:

Note:
Bonus points if you could solve it both recursively and iteratively.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null) return true;
        if (root.left == null && root.right == null) return true;
        return isMiror(root.left, root.right);
        
    }
    
    public boolean isMiror(TreeNode n1, TreeNode n2) {
        if (n1 == null && n2 == null) return true;
        if (n1 == null && n2 != null) return false;
        if (n1 != null && n2 == null) return false;
        if (n1.val != n2.val) return false;
        return isMiror(n1.left, n2.right) && isMiror(n1.right, n2.left);
    }
}

Non-recursive solution:

public class Solution {

    public boolean isSymmetric(TreeNode root) {
    if (root == null || root.left == null && root.right == null) return true;
	if (root.left != null && root.right == null) return false;
	if (root.right != null && root.left == null) return false;
	LinkedList<TreeNode> upperLevel = new LinkedList<TreeNode>(); 
    LinkedList<TreeNode> lowerLevel = new LinkedList<TreeNode>();
	TreeNode nullNode = new TreeNode(-1);
	upperLevel.add(root.left);
	upperLevel.add(root.right);

	while (upperLevel.size() != 0) {
		int length = upperLevel.size();
		for (int i = (length - 1) / 2; i >= 0; i--) {
			if (upperLevel.get(i) == nullNode && upperLevel.get(length - i - 1) == nullNode) {

			} else if (upperLevel.get(i).val == upperLevel.get(length - i - 1).val) {
				insertElement(upperLevel.get(i).right, lowerLevel, 0, nullNode);
				insertElement(upperLevel.get(i).left, lowerLevel, 0, nullNode);
				insertElement(upperLevel.get(length - i - 1).left, lowerLevel, nullNode);
				insertElement(upperLevel.get(length - i - 1).right, lowerLevel, nullNode);
			} else {
				return false;
			}			
		}
		LinkedList<TreeNode> temp = upperLevel;
		upperLevel = lowerLevel;
		lowerLevel = temp;
		lowerLevel.clear();
		
	}
	return true;
    }

	public void insertElement(TreeNode node, LinkedList<TreeNode> list, int i, TreeNode nullNode) {
		if (node == null) {
			list.add(i, nullNode);
		} else {
			list.add(i, node);
		}
	}

	public void insertElement(TreeNode node, LinkedList<TreeNode> list, TreeNode nullNode) {
		if (node == null) {
			list.add(nullNode);
		} else {
			list.add(node);
		}
	}
}

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