101-symmetricTree.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
// recursive
var isSymmetric = function(root) {
    if (!root) return true;
    return isSymmetricHelper(root.left, root.right);
};

var isSymmetricHelper = function(left, right) {
  if (!left || !right) return left === right;
  if (left.val !== right.val) return false;
  return isSymmetricHelper(left.left, right.right) && isSymmetricHelper(left.right, right.left);
};

// second try, not good.
var helper = function(left, right) {
    if (!left || !right) {
        if (!left && !right) return true;
        return false;
    }
    if ((left.val === right.val) && helper(left.left, right.right) && helper(left.right, right.left)) {
        return true;
    }
    return false;
}

// third try
var helper = function(left, right) {
    if (!left && !right) {
        return true;
    } else if (!left || !right) {
        return false;
    } else {
        if (left.val === right.val) {
            return isSymmetricHelper(left.left, right.right) && isSymmetricHelper(left.right, right.left);
        } else {
            return false;
        }
    }
}

// iterative
var isSymmetric = function(root) {
    if (!root) return true;
    var treeStack = [];
    if (root.left) {
        if (!root.right) return false;
        treeStack.push(root.left);
        treeStack.push(root.right);
    } else if (root.right) {
      return false;
    }

    while(treeStack.length > 0) {
        if (treeStack.length % 2 !== 0) return false;
        var right = treeStack.pop();
        var left = treeStack.pop();
        if (right.val !== left.val) return false;
        if (left.left) {
            if (!right.right) return false;
            treeStack.push(left.left, right.right);
        } else if (right.right) return false;

        if (right.left) {
            if (!left.right) return false;
            treeStack.push(right.left, left.right);
        } else if (left.right) return false;
    }

    return true;
};

// a second try
var isSymmetric = function(root) {
    if (!root) return true;
    var stack = [];
    if (root.left && root.right) {
        stack.push(root.left);
        stack.push(root.right);
    } else if (!root.left && !root.right) {
        return true;
    } else {
        return false;
    }

    while (stack.length > 0) {
        if (stack.length % 2 !== 0) return false;
        var left = stack.pop();
        var right = stack.pop();
        if (left.val !== right.val) return false;
        if (left.left && right.right) {
            stack.push(left.left, right.right);
        } else if ((!left.left && right.right) || (left.left && !right.right)) {
            return false;
        }
        if (right.left && left.right) {
            stack.push(right.left, left.right);
        } else if ((!right.left && left.right) || (right.left && !left.right)) {
            return false;
        }
    }

    return true;
};