225. 用队列实现栈 (二刷)

Author: w2xi

vs-lt/225.用队列实现栈.js

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+/*
+ * @lc app=leetcode.cn id=225 lang=javascript
+ *
+ * [225] 用队列实现栈
+ */
+
+// @lc code=start
+
+var MyStack = function() {
+  this.queue = [];
+  this.backup = [];
+};
+
+/** 
+ * @param {number} x
+ * @return {void}
+ */
+MyStack.prototype.push = function(x) {
+  this.queue.push(x);
+};
+
+/**
+ * @return {number}
+ */
+MyStack.prototype.pop = function() {
+  // 思路: 对 queue 执行出队操作, 当 queue 中只剩一个元素时, 队头元素就是 栈顶元素.
+  // queue:  [1, 2, 3, 4]
+  // backup: []
+  // =>
+  // queue:  [4]
+  // backup: [1, 2, 3]
+  // =>
+  // queue:  [4, 1, 2, 3]
+  // backup: []
+
+  while (this.queue.length > 1) {
+    this.backup.push(this.queue.shift());
+  }
+  while (this.backup.length > 0) {
+    this.queue.push(this.backup.shift());
+  }
+
+  return this.queue.shift();
+};
+
+/**
+ * @return {number}
+ */
+MyStack.prototype.top = function() {
+  const result = this.pop();
+  this.push(result);
+  return result;
+};
+
+/**
+ * @return {boolean}
+ */
+MyStack.prototype.empty = function() {
+  return this.queue.length === 0;
+};
+
+/**
+ * Your MyStack object will be instantiated and called as such:
+ * var obj = new MyStack()
+ * obj.push(x)
+ * var param_2 = obj.pop()
+ * var param_3 = obj.top()
+ * var param_4 = obj.empty()
+ */
+// @lc code=end
+