101. Symmetric Tree

SymmetricTreeChecker.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    static int rootValue;

    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        rootValue = root.val;

        List<Integer> leftTravasalResult = new ArrayList<>();
        travasalLeftTree(leftTravasalResult, root);
        List<Integer> rightTravasalResult = new ArrayList<>();
        travasalRightTree(rightTravasalResult, root);

        if (leftTravasalResult.size() != rightTravasalResult.size()) {
            return false;
        }

        for (int i = 0; i < leftTravasalResult.size(); i++) {
            if (leftTravasalResult.get(i) != rightTravasalResult.get(i)) {
                return false;
            }
        }

        return true;
    }
    
    private static void travasalLeftTree(List<Integer> leftTravasalResult, TreeNode node) {
        if (node == null) {
            leftTravasalResult.add(null);
            return;
        }

        leftTravasalResult.add(node.val);
        travasalLeftTree(leftTravasalResult, node.left);
        if (node.val == rootValue) {
            return;
        }
        travasalLeftTree(leftTravasalResult, node.right);
    }

    private static void travasalRightTree(List<Integer> rightTravasalResult, TreeNode node) {
        if (node == null) {
            rightTravasalResult.add(null);
            return;
        }

        rightTravasalResult.add(node.val);
        travasalRightTree(rightTravasalResult, node.right);
        if (node.val == rootValue) {
            return;
        }
        travasalRightTree(rightTravasalResult, node.left);
    }
}