[leetcode]Symmetric Tree

有意思的是还要写非递归版本。

这是递归版本：

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if (root == null) return true;
        return (isMirror(root.left, root.right));
    }
    
    public boolean isMirror(TreeNode p, TreeNode q) {
        if (p == null && q == null) return true;
        if (p != null && q != null &&
            q.val == p.val &&
            isMirror(p.left, q.right) &&
            isMirror(p.right, q.left)) {
            return true;
        }
            
        return false;
    }
}

非递归版本。

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        // Start typing your Java solution below
        // DO NOT write main() function
        
        if (root == null) return true;
        if (root.left == null && root.right == null) return true;
        if (root.left == null || root.right == null) return false;
        
        LinkedList<TreeNode> lq = new LinkedList<TreeNode>();
        LinkedList<TreeNode> rq = new LinkedList<TreeNode>();
        
        lq.offer(root.left);
        rq.offer(root.right);
        
        while (lq.size() != 0 && rq.size() != 0) {
            TreeNode ln = lq.poll();
            TreeNode rn = rq.poll();
            
            if (ln == null && rn == null) continue;
            if (ln == null || rn == null) return false;
            if (ln.val != rn.val) return false;
            
            lq.offer(ln.left);
            lq.offer(ln.right);
            rq.offer(rn.right);
            rq.offer(rn.left);
        }
        
        return true;
    }
}