算法1

代码

class Solution {
    public boolean isSymmetric(TreeNode root) {
        if (root == null){
            return true;
        }
        return isSymmetric0(root.left,root.right);
    }

    private boolean isSymmetric0(TreeNode p1, TreeNode p2){
        if (p1 == null && p2 == null){
            return true;
        }
        if (p1 == null || p2 == null){
            return false;
        }
        return p1.val == p2.val && isSymmetric0(p1.left,p2.right) && isSymmetric0(p1.right,p2.left);
    }
}

算法2

代码

boolean isSymmetricalDFS(TreeNode pRoot) {
        if(pRoot == null) return true;
        Stack<TreeNode> s = new Stack<>();
        s.push(pRoot.left);
        s.push(pRoot.right);
        while(!s.empty()) {
            TreeNode right = s.pop();//成对取出
            TreeNode left = s.pop();
            if(left == null && right == null) continue;
            if(left == null || right == null) return false;
            if(left.val != right.val) return false;
            //成对插入
            s.push(left.left);
            s.push(right.right);
            s.push(left.right);
            s.push(right.left);
        }
        return true;
    }