Leetcode402 remove K digits

Title Description ： Give you a nonnegative integer expressed as a string num And an integer k , Remove the k Digit number , Keep the remaining numbers to a minimum . Please return the smallest number as a string .

Example ：
Input ：num = “1432219”, k = 3
Output ：“1219”
explain ： Remove three numbers 4, 3, and 2 Form a new minimum number 1219 .

analysis ：
When the... Of the string i The number corresponding to bit is greater than i+1 Number corresponding to bit , We delete No i The number of digits is reserved for i+1 The number of . Use the heap to constantly store the obtained numbers , This ensures that the heap maintains an increasing order from the bottom of the heap to the top of the heap , Then we will get the newly acquired number i Compare the number corresponding to bit with the number on the top of the heap , Make sure the top of the heap is a small number . Only to delete k A digital , Then loop all the remaining numbers into the queue . Then create a dynamic string , Get each string from the bottom of the heap and transfer it to the dynamic string .

Knowledge point ：
1: Create a heap with a value of characters

Deque<Character> deque = new LinkedList<Character>();

2: Get the character corresponding to the string

char digit = num.charAt(i);

3: Determine whether the heap is empty

deque.isEmpty()

4: Add or remove elements to the heap

deque.offerLast(digit);
deque.pollLast();
deque.pollFirst();

deque.peekLast()
 Get the top element , But don't delete 

5: Create a dynamic string

 StringBuilder ret = new StringBuilder();

Add an element to a string

ret.append(digit);