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

解法參考花花的影片

/**
 * 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
}