solve #155

Author: aylei

src/lib.rs

@@ -149,3 +149,4 @@ mod n0151_reverse_words_in_a_string;
 mod n0152_maximum_product_subarray;
 mod n0153_find_minimum_in_rotated_sorted_array;
 mod n0154_find_minimum_in_rotated_sorted_array_ii;
+mod n0155_min_stack;

src/n0155_min_stack.rs

@@ -0,0 +1,114 @@
+/**
+ * [155] Min Stack
+ *
+ * 
+ * Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
+ * 
+ * 
+ * push(x) -- Push element x onto stack.
+ * 
+ * 
+ * pop() -- Removes the element on top of the stack.
+ * 
+ * 
+ * top() -- Get the top element.
+ * 
+ * 
+ * getMin() -- Retrieve the minimum element in the stack.
+ * 
+ * 
+ * 
+ * 
+ * Example:<br />
+ * 
+ * MinStack minStack = new MinStack();
+ * minStack.push(-2);
+ * minStack.push(0);
+ * minStack.push(-3);
+ * minStack.getMin();   --> Returns -3.
+ * minStack.pop();
+ * minStack.top();      --> Returns 0.
+ * minStack.getMin();   --> Returns -2.
+ * 
+ * 
+ */
+pub struct Solution {}
+
+// submission codes start here
+
+/*
+ 这题居然是 easy... 我怀疑人生了, getMin() 怎么能做到常数时间? Heap 也是 LogN 啊
+
+ 看了最高票解之后...........天哪, 我可太菜了
+
+ 核心思想是保证每次 pop 时都能以常数时间更新最小值, 这就需要在空间上以某种方式记录下来
+
+ 那一种做法就是存储每个元素和最小值之间的差值, 这样 pop 的时候就能不断还原出原始值
+
+ 另一种更直观的做法就是每次入栈 min 时, 都把前一个 min (当前第二小的数字) 放在它前面, 作为记录
+ */
+struct MinStack {
+    vec: Vec<i32>,
+    min: i32,
+}
+
+impl MinStack {
+
+    /** initialize your data structure here. */
+    pub fn new() -> Self {
+        MinStack{
+            vec: Vec::new(),
+            min: i32::max_value(),
+        }
+    }
+    
+    pub fn push(&mut self, x: i32) {
+        if x <= self.min {
+            self.vec.push(self.min);
+            self.min = x;
+        }
+        self.vec.push(x);
+    }
+
+    pub fn pop(&mut self) {
+        if self.vec.pop().unwrap() == self.min {
+            self.min = self.vec.pop().unwrap();
+        }
+    }
+    
+    pub fn top(&self) -> i32 {
+        *self.vec.last().unwrap()
+    }
+    
+    pub fn get_min(&self) -> i32 {
+        self.min
+    }
+}
+
+/**
+ * Your MinStack object will be instantiated and called as such:
+ * let obj = MinStack::new();
+ * obj.push(x);
+ * obj.pop();
+ * let ret_3: i32 = obj.top();
+ * let ret_4: i32 = obj.get_min();
+ */
+
+// submission codes end
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    pub fn test_155() {
+        let mut min_stack = MinStack::new();
+        min_stack.push(-2);
+        min_stack.push(0);
+        min_stack.push(-3);
+        assert_eq!(min_stack.get_min(), -3);   // --> Returns -3.
+        min_stack.pop();
+        assert_eq!(min_stack.top(), 0);      // --> Returns 0.
+        assert_eq!(min_stack.get_min(), -2);   // --> Returns -2.[]
+    }
+}