题解 | #牛群的树形结构展开#

function flattenTree( root ) {
    // write code here
    if(root == null) return null

    flattenTree(root.left)
    flattenTree(root.right)

    let left = root.left
    let right = root.right

    root.left = null
    root.right = left
    let cur = root
    while(cur.right != null) {
        cur = cur.right
    }
    cur.right = right
    return root
}