LeetCode 101: Symmetric Tree

/**
 * 101. Symmetric Tree
 * 1. Time:O(n)  Space:O(n)
 * 2. Time:O(n)  Space:O(n)
 */

// 1. Time:O(n)  Space:O(n)
class Solution {
    public boolean isSymmetric(TreeNode root) {
        return isMirror(root,root);
    }
    
    private boolean isMirror(TreeNode p, TreeNode q){
        if(p==null && q==null) return true;
        if(p==null || q==null) return false;
        return p.val==q.val && isMirror(p.left,q.right) && isMirror(p.right,q.left);
    }
}

// 2. Time:O(n)  Space:O(n)
class Solution {
    public boolean isSymmetric(TreeNode root) {
        Stack<TreeNode> s = new Stack<>();
        s.push(root);
        s.push(root);
        while(!s.isEmpty()){
            TreeNode p = s.pop();
            TreeNode q = s.pop();
            if(p==null && q==null) continue;
            if(p==null || q==null) return false;
            if(p.val!=q.val) return false;
            s.push(p.left);
            s.push(q.right);
            s.push(p.right);
            s.push(q.left);
        }
        return true;
    }
}