[LeetCode 101] Symmetric Tree

题目链接：symmetric-tree

import java.util.LinkedList;

//判断该二叉树是否是对称二叉树

/**
 * 
		Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
		
		For example, this binary tree is symmetric:
		
		    1
		   / 		  2   2
		 / \ / 		3  4 4  3
		But the following is not:
		    1
		   / 		  2   2
		   \   		   3    3
		Note:
		Bonus points if you could solve it both recursively and iteratively.


 *
 */

public class SymmetricTree {
	public class TreeNode {
		int val;
		TreeNode left;
		TreeNode right;

		TreeNode(int x) {
			val = x;
		}
	}
	//解法一：递归版
//	192 / 192 test cases passed.
//	Status: Accepted
//	Runtime: 248 ms
//	Submitted: 0 minutes ago
	
    public boolean isSymmetric(TreeNode root) {
    	if(root == null) return true;
    	return isSymmetric(root.left, root.right);
    	
    }
    public boolean isSymmetric(TreeNode p, TreeNode q) {
    	if(p == null && q == null) return true;
    	if(p == null || q == null) return false;
    	return p.val == q.val 
    			&& isSymmetric(p.left, q.right)
    			&& isSymmetric(p.right, q.left);
    	 
    	
    }
    //解法二：遍历版
//	192 / 192 test cases passed.
//	Status: Accepted
//	Runtime: 240 ms
//	Submitted: 0 minutes ago
	
    public boolean isSymmetric1(TreeNode root) {
    	
    	if(root == null) return true;
    	LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
    	queue.add(root);
    	while(!queue.isEmpty()) {
    		int levelLen = queue.size();
    		
    		//判断 每层是否对称
    		int p = 0;
    		int q = levelLen - 1;
    		while(p < q) {
    			TreeNode nodeP = queue.get(p);
    			TreeNode nodeQ = queue.get(q);
    			if(nodeP == null && nodeQ != null ) return false;
    			else if(nodeP != null && nodeQ == null ) return false;
    			else if( nodeP != null && nodeQ != null && nodeP.val != nodeQ.val) return false;
    		//	else if( nodeP == null && nodeQ == null) 不处理;
    			p ++;
    			q --;
    		}
    		
    		//子节点入队
    		for (int i = 0; i < levelLen; i++) {
				TreeNode node = queue.removeFirst();
				if(node != null ) {
					queue.add(node.left);
					queue.add(node.right);
				}
			}
    	}
        return true;
    }	
	public static void main(String[] args) {
		// TODO Auto-generated method stub

	}

}

[LeetCode 101] Symmetric Tree

原文：http://blog.csdn.net/ever223/article/details/44535995