101.IsSymmetric.cs

// 101. Symmetric Tree

// Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

// Example 1:
// Input: root = [1,2,2,3,4,4,3]
// Output: true
// Example 2:
// Input: root = [1,2,2,null,3,null,3]
// Output: false

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public bool IsSymmetric(TreeNode root)
    {
        if (root == null)
            return true;
        return CheckInvertTree(root.left, root.right);
    }
    
    public bool CheckInvertTree(TreeNode root1, TreeNode root2)
    {
        if (root1 == null && root2 == null )
            return true;
        if (root1 == null || root2 == null)
            return false;
        if (root1.val != root2.val)
            return false;
        return CheckInvertTree(root1.left, root2.right) &&
        CheckInvertTree(root1.right, root2.left);
    }
}