LC.P687[最长同值路径]

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 ans;

    public int longestUnivaluePath(TreeNode root) {
        dfs(root);
        return ans;
    }

    private int dfs(TreeNode root) {
        if (root == null) return -1; // 下面+1后，对于叶子节点就刚好为0
        int left = dfs(root.left) + 1; // 左子树最大链长+1
        int right = dfs(root.right) + 1; // 右子树最大链长+1
        if (root.left != null && root.left.val != root.val) left = 0; // 链长视为0
        if (root.right != null && root.right.val != root.val) right = 0; // 链长视为0
        ans = Math.max(ans, left + right);
        return Math.max(left, right); // 当前子树最大链长
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$