226. Invert Binary Tree

트리를 좌우대칭으로 바꾼다.
heap정렬을 하면서, 각 층에서 node들을 stack에 넣고 꺼낸다.

재귀

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
// 1. root가 없으면 return null;
// 2. create temp cariable and assign to root.left.
// 3. change root.left to equql root.right
// 4. change root.right to qqual ro temp
// 5. InverTree(root.left)
// 6. InverTree(root.right)

// Time Complexity: O(n)
// Space Complexity: O(n)

var invertTree = function(root) {
    if (!root) return null;
    
    let temp = root.left;
    root.left = root.right;
    root.right = temp;
    
    invertTree(root.left);
    invertTree(root.right);
    
    return root;
};