687. Longest Univalue Path

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687. Longest Univalue Path

题目描述

Given a binary tree, find the length of the longest path where each node in the path has the same value. This path may or may not pass through the root.

Note: The length of path between two nodes is represented by the number of edges between them.

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Example 1:
Input:
              5
             / \
            4   5
           / \   \
          1   1   5
Output:
2
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Example 2:
Input:
              1
             / \
            4   5
           / \   \
          4   4   5
Output:
2

Note: The given binary tree has not more than 10000 nodes. The height of the tree is not more than 1000.

题目大意

求二叉树节点值全部相等的最长路径，路径不一定通过根节点。

解题思路

递归遍历左右子树，寻找左右子树节点值全部相等的最长路径。

代码

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func longestUnivaluePath(_ root: TreeNode?) -> Int {
        var ans = 0
        func longestPath(_ root: TreeNode?) -> Int {
            guard let tree = root else { return 0 }
            let lc = longestPath(tree.left)
            let rc = longestPath(tree.right)
            var ll = 0, rl = 0
            if let left = tree.left, left.val == tree.val { ll = lc + 1 }
            if let right = tree.right, right.val == tree.val { rl = rc + 1 }
            ans = max(rl + ll, ans)
            return max(rl, ll)
        }
        longestPath(root)
        return ans
    }
}
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