965. Univalued Binary Tree 整个一样的二叉树

A binary tree is univalued if every node in the tree has the same value.

Return true if and only if the given tree is univalued.

Example 1:

Input: [1,1,1,1,1,null,1]
Output: true

Example 2:

Input: [2,2,2,5,2]
Output: false

反正就是尽量用traverse。少用bst的模板（哪怕是bst），因为没人用

class Solution {
    int val = -1;
    public boolean isUnivalTree(TreeNode root) {
        if (root == null) return true;
        if (val < 0) val = root.val;
        return root.val == val && isUnivalTree(root.left)  && isUnivalTree(root.right);
    }
}

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来源： https://www.cnblogs.com/immiao0319/p/15028551.html