一、题目

  Given a binary tree, return the tilt of the whole tree.

  The tilt of a tree node is defined as the absolute difference between the sum of all left subtree node values and the sum of all right subtree node values. Null node has tilt 0.

  The tilt of the whole tree is defined as the sum of all nodes’ tilt.

  Example:

  Input: 
          1
        /   \
       2     3
  Output: 1
  Explanation: 
  Tilt of node 2 : 0
  Tilt of node 3 : 0
  Tilt of node 1 : |2-3| = 1
  Tilt of binary tree : 0 + 0 + 1 = 1

二、题目大意

   树节点的倾斜度定义为所有左子树节点值的总和与所有右子树节点值的总和之间的绝对差，而整个树的倾斜度定义为所有节点倾斜度之和，求出整个树的倾斜度。

三、解题思路

  递归计算左右节点的差值，但是在递归的过程中，需要不断向上返回该节点的子节点之和。

四、代码实现

const findTilt = root => {
  function helper (root) {
    if (!root) {
      return 0
    }
    const left = helper(root.left)
    const right = helper(root.right)

    sum += Math.abs(left - right)
    return root.val + left + right
  }
  let sum = 0
  helper(root)
  return sum
}

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