LC.P965[单值二叉树]

方法一：DFS

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    int val = -1;

    public boolean isUnivalTree(TreeNode root) {
        if (val == -1) val = root.val;
        if (root == null) return true;
        if (root.val != val) return false;
        return isUnivalTree(root.left) && isUnivalTree(root.right);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$