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  • leetcode543

    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
              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.
     
     
    树的递归
    问题转化:对每个节点,计算左深度+右深度的值,就是经过该节点的最长path的edge数。
    那如果对每个节点都得到过这个值,并且和全局变量打擂台,那就得到答案了呀。经过某个节点得到的最长path对应的长度就被存储在全局变量里了。
    递归函数返回当前节点的深度给父亲用。递归函数内借助孩子打擂台。 
     
    实现:
    /**
     * Definition for a binary tree node.
     * public class TreeNode {
     *     int val;
     *     TreeNode left;
     *     TreeNode right;
     *     TreeNode(int x) { val = x; }
     * }
     */
    class Solution {
        int maxLen = 0;
        public int diameterOfBinaryTree(TreeNode root) {
            calHelper(root);
            return maxLen;
        }
        
        /*
        * return the current depth. leaf node has depth of 1.
        */
        private int calHelper(TreeNode root) {
            if (root == null) {
                return 0;
            }
            int leftDepth = calHelper(root.left);
            int rightDepth = calHelper(root.right);
            maxLen = Math.max(maxLen, leftDepth + rightDepth);
            return Math.max(leftDepth, rightDepth) + 1;
        }
    }
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  • 原文地址:https://www.cnblogs.com/jasminemzy/p/9771834.html
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