LeetCode 965 Univalued Binary Tree

给予一棵树:
        1
       / \
      1   1
     / \
    1   1
应当返回 `true`

给予一棵树:
        1
       / \
      1   1
     / \
    5   1
应当返回 `false`

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isUnivalTree(TreeNode root) {
        int i = root.val;
        
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if (node.val != i) {
                return false;
            }
            if (node.left != null) {
                stack.push(node.left);
            }
            if (node.right != null) {
                stack.push(node.right);
            }
        }
        
        return true;
    }
}