leetcode-572-subtree-of-another-tree.swift

/**

Copyright (c) 2020 David Seek, LLC

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OR OTHER DEALINGS IN THE SOFTWARE.





https://leetcode.com/problems/subtree-of-another-tree/


Given two non-empty binary trees s and t, check whether tree t has exactly the same structure and node values with a subtree of s. A subtree of s is a tree consists of a node in s and all of this node's descendants. The tree s could also be considered as a subtree of itself.

Example 1:
Given tree s:

     3
    / \
   4   5
  / \
 1   2
Given tree t:
   4 
  / \
 1   2
Return true, because t has the same structure and node values with a subtree of s.
 

Example 2:
Given tree s:

     3
    / \
   4   5
  / \
 1   2
    /
   0
Given tree t:
   4
  / \
 1   2
Return false.

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of nodes in s.
Space complexity O(1) we're not using any extra space.
*/
func isSubtree(_ s: TreeNode?, _ t: TreeNode?) -> Bool {

    // Without no s tree, nothing to check
    guard let s = s else {
        return false
    }
    
    // Check if t is a subtree of s
    guard getEquality(of: s, and: t) else {

        // If not, continue on the left and on the right
        return isSubtree(s.left, t) || isSubtree(s.right, t)
    }
    
    return true
}

private func getEquality(of s: TreeNode?, and t: TreeNode?) -> Bool {
    
    // If either node is nil...
    if s == nil || t == nil {
        
        // ... both need to be nil
        return s == nil && t == nil
    
    // If the current nodes match
    } else if s!.val == t!.val {
        
        // Then all other children need to match as well
        return getEquality(of: s!.left, and: t!.left) && getEquality(of: s!.right, and: t!.right)
    } 
    
    // Otherwise it's not a subtree
    return false
}