leetcode-965 Univalued Binary Tree

965. Univalued Binary Tree

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:

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

Example 2:

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

Note:

  1. The number of nodes in the given tree will be in the range [1, 100].
  2. Each node’s value will be an integer in the range [0, 99].

Analyse

判断一棵树是不是所有节点的值都相同

层序遍历

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bool isUnivalTree(TreeNode* root) {
    int value = root->val;

    queue<TreeNode*> que;
    que.push(root);

    while (!que.empty())
    {
        TreeNode* node = que.front();
        que.pop();

        if (node->val != value)
        {
            return false;
        }
        else
        {
            if (node->left)
            {
                que.push(node->left);
            }

            if (node->right)
            {
                que.push(node->right);
            }
        }
    }

    return true;
}

递归

分成两个子问题来判断，左子树和右子树是否是UnivalTree

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bool isUnivalTree(TreeNode* root)
{
    return (root->left == nullptr || (root->left->val == root->val && isUnivalTree(root->left)))
           &&
           (root->right == nullptr || (root->right->val == root->val && isUnivalTree(root->right)));
}