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155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(int val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

You must implement a solution with O(1) time complexity for each function.

 

Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

 

Constraints:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

---

SOL：

參考李根逸博士的影片，使用一個mins array來記錄在每個data位置的極小值。

不能只記整個stack的最小值，因為pop之後可能會把最小值pop掉。

【C 語言的 LeetCode 30 天挑戰】第十天 (Min Stack)

typedef struct {
    int *data;
    int *mins;
    int size;
} MinStack;


MinStack* minStackCreate() {
    MinStack *s = malloc(sizeof(MinStack));
    s->data = NULL;
    s->mins = NULL;
    s->size = 0;
    return s;
}

void minStackPush(MinStack* obj, int val) {
    obj->data = realloc(obj->data, (sizeof(int)*(obj->size+1)));
    obj->mins = realloc(obj->mins, (sizeof(int)*(obj->size+1)));
    obj->data[obj->size]=val;
    if((obj->size==0) || (obj->mins[obj->size-1]>val))
        obj->mins[obj->size]=val;
    else
        obj->mins[obj->size]=obj->mins[obj->size-1];
    obj->size++;
}

void minStackPop(MinStack* obj) {
    obj->size--;
}

int minStackTop(MinStack* obj) {
    return obj->data[obj->size-1];
}

int minStackGetMin(MinStack* obj) {
    return obj->mins[obj->size-1];
}

void minStackFree(MinStack* obj) {
    free(obj->data);
    free(obj->mins);
    free(obj);
}

/**
 * Your MinStack struct will be instantiated and called as such:
 * MinStack* obj = minStackCreate();
 * minStackPush(obj, val);
 
 * minStackPop(obj);
 
 * int param_3 = minStackTop(obj);
 
 * int param_4 = minStackGetMin(obj);
 
 * minStackFree(obj);
*/

 

此外，影片中也有大致講一下 '.' 跟 '->'的用法差異，這個問題也困擾我好久，剛好老師一併講了。

Struct 用 '.' ；Struct pointer 用 '->'

(*fooPtr).yy 等價於 fooPtr->yy：'->' 可以用 '*' 跟 '.'來表示，不過有點噁心，所以才用 '->' 這個語法糖。