symmetricTree.js

// Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

// For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

//     1
//    / \
//   2   2
//  / \ / \
// 3  4 4  3

// But the following [1,2,2,null,3,null,3] is not:
//     1
//    / \
//   2   2
//    \   \
//    3    3

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