如果二叉树每个节点都具有相同的值，那么该二叉树就是单值二叉树。

只有给定的树是单值二叉树时，才返回 true；否则返回 false。

示例 1：

输入：[1,1,1,1,1,null,1]
输出：true

递归

class Solution {
		public boolean isUnivalTree(TreeNode root) {

			return preOrder(root, root.val);
		}

		private boolean preOrder(TreeNode root, int target) {

			if (root.val != target) {
				return false;
			}

			boolean l = true;
			if (root.left != null) {
				l = preOrder(root.left, target);
			}

			boolean r = true;
			if (root.right != null) {
				r = preOrder(root.right, target);
			}

			return l && r;
		}
	}

迭代

class Solution2 {
		public boolean isUnivalTree(TreeNode root) {

			boolean res = true;

			int target = root.val;

			Stack<TreeNode> stack = new Stack<>();

			while (null != root || !stack.isEmpty()) {

				while (null != root) {

					if (target != root.val) {
						res = false;

					}

					stack.add(root.right);
					root = root.left;
				}

				root = stack.pop();
			}

			return res;

		}

	}
= root.left;
				}

				root = stack.pop();
			}

			return res;

		}

	}