Kotlin Leetcode - 101. Symmetric Tree

題目連接

class Solution {  
    fun isSymmetric(root: TreeNode?): Boolean {  
    }  
}

解題思路

這題是樹的處理

我們可以用遞迴的方式進行比較

• 如果 root 是 null，回傳 true
• 如果左邊和右邊都是 null，回傳 true
• 如果左邊的值和右邊的值不同，回傳 false
• 遞迴比較
  • 左邊 node 的左邊，和右邊 node 的右邊
  • 左邊 node 的右邊，和右邊 node 的左邊

Kotlin 參考解答

class Solution {
    fun isSymmetric(root: TreeNode?): Boolean {
        if (root == null) { return true }
        return isSubtreeSymmetric(root.left, root.right)
    }
    
    private fun isSubtreeSymmetric(left: TreeNode?, right: TreeNode?): Boolean {
        if (left == null && right == null) {
            return true
        }
        
        if (left?.`val` != right?.`val`) {
            return false
        }
        
        return isSubtreeSymmetric(left?.left, right?.right)
            && isSubtreeSymmetric(left?.right, right?.left)
    }
}

用 when 的寫法

class Solution {
    fun isSymmetric(root: TreeNode?): Boolean {
        if (root == null) { return true }
        return isSubtreeSymmetric(root.left, root.right)
    }

    private fun isSubtreeSymmetric(left: TreeNode?, right: TreeNode?): Boolean =
        when {
            left == null && right == null ->  true
            left?.`val` != right?.`val` -> false
            else -> isSubtreeSymmetric(left?.left, right?.right)
                && isSubtreeSymmetric(left?.right, right?.left)
        }
    
}

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