leetcode543. diameter of binary tree

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example:
Given a binary tree

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5

    1
   / 
  2   3
 /      
4   5

Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

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

计算左右子树最长路径，同时找出一个左右子树路径之和最大的节点。

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private int ans = 0;
public int (TreeNode root) {
    getDiameter(root);
    return ans;
}
private int getDiameter(TreeNode root) {
    if (root == null) return 0;
    int l = getDiameter(root.left);
    int r = getDiameter(root.right);
    ans = Math.max(ans, l + r);
    return Math.max(l, r) + 1;
}