Symmetric Tree

isSymmetric.java

/*
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
   
Algorithm:
This problem will be good for topdown approach from a tree
1. Check all the situation execpt when both value are same
2. Repeat the recusion for left and right
3. Return the value for left and right and left.val == right.val

*/
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
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 isSymmetricHelper(root.left, root.right);
    }
    
    public boolean isSymmetricHelper(TreeNode left, TreeNode right) {
        if(left == null && right == null) return true;
        if(left == null) return false;
        if(right == null) return false;
        if(left.val != right.val) return false;
        
        boolean leftCheck = isSymmetricHelper(left.left, right.right);
        boolean rightCheck = isSymmetricHelper(left.right, right.left);
        
        return leftCheck && rightCheck && (left.val == right.val);
    }
}