简化Peek函数，复用Pop

problems/0232.用栈实现队列.md

@@ -275,15 +275,11 @@ func (this *MyQueue) Pop() int {
 
 /** Get the front element. */
 func (this *MyQueue) Peek() int {
-    for len(this.stack) != 0 {
-        val := this.stack[len(this.stack)-1]
-        this.stack = this.stack[:len(this.stack)-1]
-        this.back = append(this.back, val)
-    }
-    if len(this.back) == 0 {
+    val := this.Pop()
+    if val == 0 {
         return 0
     }
-    val := this.back[len(this.back)-1]
+    this.back = append(this.back, val)
     return val
 }