LC.P543[二叉树的直径]

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 diameterOfBinaryTree(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
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:$O(n)$