leetcode-maximum-width-of-binary-tree

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题目大意

　　https://leetcode.com/contest/leetcode-weekly-contest-46/problems/maximum-width-of-binary-tree/

　　求树的同层横向最大距离。

题目分析

　　思路参考了 https://discuss.leetcode.com/topic/100149/c-java-clean-code-with-explanation

　　利用DFS计算，要为每个节点引入id（类似于二叉树用数组存储的思路，id的节点左右子节点存放在2 * id和2 * id + 1位置），方便后续计算距离。

代码

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private int ans;
    
    private void dfs(TreeNode cur, int level, int idx, List<Integer> leftIds) {
        if (cur == null) {
            return;
        }
        if (leftIds.size() == level) {
            leftIds.add(idx);
        }
        ans = Math.max(ans, idx - leftIds.get(level) + 1);
        dfs(cur.left, level + 1, idx * 2, leftIds);
        dfs(cur.right, level + 1, idx * 2 + 1, leftIds);
    }
    
    public int widthOfBinaryTree(TreeNode root) {
        if (root == null) {
            return 0;
        }
        ans = 1;
        List<Integer> leftIds = new ArrayList<>();
        dfs(root, 0, 1, leftIds);
        return ans;
    }
}

　　时间复杂度：O(n)，空间复杂度：o(h)