Kotlin之Elvis 操作符

fun <T> Iterable<T>.contentHashCode(): Int {
    return fold(1) {
        hash, element ->
        hash * 31 + (element?.hashCode() ?: 0)
    }
}

val v = a?.b ?: c

var temp = if(a != null) a.b else null
val v = if(temp != null) temp else c

val v = if(a == null || a.b == null) c else a.b

private fun preOrderLoop(
    root: Node? = this.root,
    list: MutableList<Int> = mutableListOf()
): List<Int> {
    val stack = ArrayDeque<Node>()
    stack.push(root ?: return list)

    while (stack.isNotEmpty()) {
        val nodeNow = stack.pop()!!
        list += nodeNow.value
        nodeNow.right?.let { stack.push(it) }
        nodeNow.left?.let { stack.push(it) }
    }
    return list
}

if(root == null) return list
stack.push(root)

inline infix fun <T : Any> T?.ifNull(block: (T?) -> T): T {
    if (this == null) {
        return block(this)
    }
    return this
}

val file = java.io.File("C:\\FakeFile")
val parent = file.parent ifNull {
    // do something here
}