979. Distribute Coins in Binary Tree

题目描述

Given the root of a binary tree with N nodes, each node in the tree has node.val coins, and there are N coins total.

In one move, we may choose two adjacent nodes and move one coin from one node to another. (The move may be from parent to child, or from child to parent.)

Return the number of moves required to make every node have exactly one coin.

方法思路

class Solution {
    //Runtime: 4 ms, faster than 100.00%
    //Memory Usage: 37.2 MB, less than 89.78%
    int ans;
    public int distributeCoins(TreeNode root) {
        ans = 0;
        dfs(root);
        return ans;
    }

    public int dfs(TreeNode node) {
        if (node == null) return 0;
        int L = dfs(node.left);
        int R = dfs(node.right);
        ans += Math.abs(L) + Math.abs(R);
        //当子节点计算结束后，更新需要推给父节点的硬币数。
        return node.val + L + R - 1;
    }
}