979. Distribute Coins in Binary Tree ⭐

Medium
You are given the `root` of a binary tree with `n` nodes where each `node` in the tree has `node.val` coins. There are `n` coins in total throughout the whole tree.
In one move, we may choose two adjacent nodes and move one coin from one node to another. A move may be from parent to child, or from child to parent.
Return the minimum number of moves required to make every node have exactly one coin.
Example 1:
Input: root = [3,0,0]
Output:
2
Explanation:
From the root of the tree, we move one coin to its left child, and one coin to its right child.
Example 2:
Input: root = [0,3,0]
Output:
3
Explanation:
From the left child of the root, we move two coins to the root [taking two moves]. Then, we move one coin from the root of the tree to the right child.
Constraints:
• The number of nodes in the tree is `n`.
• `1 <= n <= 100`
• `0 <= Node.val <= n`
• The sum of all `Node.val` is `n`.

解題

/**
* Definition for a binary tree node.
* type TreeNode struct {
* Val int
* Left *TreeNode
* Right *TreeNode
* }
*/
func distributeCoins(root *TreeNode) int {
ans := 0
var balance func(*TreeNode) int
balance = func(root *TreeNode) int{
if root == nil { return 0 }
l := balance(root.Left)
r := balance(root.Right)
ans += abs(l) + abs(r)
return l + r + root.Val - 1
}
balance(root)
return ans
}
func abs(a int) int {
if a < 0 { return -a }
return a
}