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docs: 新增No.101、No.102题解

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`17`	`17`	`- [No.70 爬楼梯](note/No70_climb-stairs.md)`
`18`	`18`	`- [No.88 合并两个有序数组](note/No88_merge.md)`
`19`	`19`	`- [No.98 验证二叉搜索树](note/No98_is-valid-BST.md)`
	`20`	`+- [No.101 对称二叉树](note/No101_is-symmetric.md)`
	`21`	`+- [No.102 二叉树的层次遍历](note/No102_level-order.md)`
`20`	`22`	`- [No.104 二叉树的最大深度](note/No104_max-depth.md)`
`21`	`23`	`- [No.121 买卖股票的最佳时机](note/No121_max-profit.md)`
`22`	`24`	`- [No.122 买卖股票的最佳时机 II](note/No122_max-profit.md)`

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	`1`	`+# No.101 对称二叉树`
	`2`	`+`
	`3`	+难度:`easy`
	`4`	`+`
	`5`	`+`
	`6`	`+`
	`7`	`+给定一个二叉树,检查它是否是镜像对称的。`
	`8`	`+`
	`9`	`+例如,二叉树 [1,2,2,3,4,4,3] 是对称的。`
	`10`	+```
	`11`	`+ 1`
	`12`	`+ / \`
	`13`	`+ 2 2`
	`14`	`+ / \ / \`
	`15`	`+3 4 4 3`
	`16`	+```
	`17`	`+但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:`
	`18`	+```
	`19`	`+ 1`
	`20`	`+ / \`
	`21`	`+ 2 2`
	`22`	`+ \ \`
	`23`	`+ 3 3`
	`24`	+```
	`25`	`+说明:`
	`26`	`+`
	`27`	`+如果你可以运用递归和迭代两种方法解决这个问题,会很加分。`
	`28`	`+`
	`29`	`+## 解题思路`
	`30`	`+`
	`31`	`+使用递归的思路,同时判断左右是否相等.`
	`32`	`+`
	`33`	`+代码如下:`
	`34`	`+`
	`35`	+```javascript
	`36`	`+/**`
	`37`	`+ * Definition for a binary tree node.`
	`38`	`+ * function TreeNode(val) {`
	`39`	`+ * this.val = val;`
	`40`	`+ * this.left = this.right = null;`
	`41`	`+ * }`
	`42`	`+ */`
	`43`	`+/**`
	`44`	`+ * @param {TreeNode} root`
	`45`	`+ * @return {boolean}`
	`46`	`+ */`
	`47`	`+var isSymmetric = function(root) {`
	`48`	`+ return isMirror(root,root);`
	`49`	`+};`
	`50`	`+`
	`51`	`+var isMirror = function(l,r) {`
	`52`	`+ if (l == null && r == null) return true;`
	`53`	`+ if (l == null \|\| r == null) return false;`
	`54`	`+ return (l.val == r.val)`
	`55`	`+ && isMirror(l.right, r.left)`
	`56`	`+ && isMirror(l.left, r.right);`
	`57`	`+}`
	`58`	+```

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	`1`	`+# No.102 二叉树的层次遍历`
	`2`	`+`
	`3`	+难度:`middle`
	`4`	`+`
	`5`	`+给定一个二叉树,返回其按层次遍历的节点值。 (即逐层地,从左到右访问所有节点)。`
	`6`	`+`
	`7`	`+例如:`
	`8`	`+给定二叉树: [3,9,20,null,null,15,7],`
	`9`	`+`
	`10`	+```
	`11`	`+ 3`
	`12`	`+ / \`
	`13`	`+ 9 20`
	`14`	`+ / \`
	`15`	`+ 15 7`
	`16`	+```
	`17`	`+返回其层次遍历结果:`
	`18`	`+`
	`19`	+```
	`20`	`+[`
	`21`	`+ [3],`
	`22`	`+ [9,20],`
	`23`	`+ [15,7]`
	`24`	`+]`
	`25`	+```
	`26`	`+`
	`27`	`+## 解题思路`
	`28`	`+`
	`29`	`+使用队列的思路,宽度优先搜索,每次遍历都将节点入队列,并加入到输出的数组中。`
	`30`	`+`
	`31`	`+代码如下:`
	`32`	`+`
	`33`	+```javascript
	`34`	`+/**`
	`35`	`+ * Definition for a binary tree node.`
	`36`	`+ * function TreeNode(val) {`
	`37`	`+ * this.val = val;`
	`38`	`+ * this.left = this.right = null;`
	`39`	`+ * }`
	`40`	`+ */`
	`41`	`+/**`
	`42`	`+ * @param {TreeNode} root`
	`43`	`+ * @return {number[][]}`
	`44`	`+ */`
	`45`	`+var levelOrder = function(root) {`
	`46`	`+ if (root == null) {`
	`47`	`+ return [];`
	`48`	`+ }`
	`49`	`+ let queue = [root];`
	`50`	`+ let res = [];`
	`51`	`+ while (queue.length) {`
	`52`	`+ let arr = [];`
	`53`	`+ let length = queue.length;`
	`54`	`+ for (let i = 0; i < length; i++) {`
	`55`	`+ let front = queue.shift();`
	`56`	`+ arr.push(front.val);`
	`57`	`+ if (front.left)`
	`58`	`+ queue.push(front.left);`
	`59`	`+ if (front.right)`
	`60`	`+ queue.push(front.right);`
	`61`	`+ }`
	`62`	`+ res.push(arr);`
	`63`	`+ }`
	`64`	`+ return res;`
	`65`	`+};`
	`66`	+```

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