2018 cs61b examprep05

https://sp18.datastructur.es/materials/discussion/examprep05.pdf
2 Use them!
a.
类似lc的第一题，直接用代码了

    import java.util.HashMap;

    class Solution {
        public int[] twoSum(int[] nums, int target) {
            int[] res = new int[2];
            HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
            for(int i=0;i<nums.length;i++){
                if(map.containsKey(nums[i])){
                    res[0]=map.get(nums[i]);
                    res[1]=i;
                    return res;
                }
                map.put(target-nums[i], i);
            }
            return res;
        }
    }

b.
先用map把每个word是次数记录下来，然后用优先队列（自己建立排序方法），最后输出。

3
a.

    import java.util.Stack;

    public class Deque<Item> {
        private final Stack<Item> a;
        private final Stack<Item> b;

        public Deque() {
            a = new Stack<Item>();
            b = new Stack<Item>();
        }

        public void push(Item t) {
            while (!a.empty()) {
                b.push(a.pop());
            }
            a.push(t);
            while (!b.empty()) {
                a.push(b.pop());
            }
        }

        public Item poll() {
            return a.pop();
        }
    }

b.在上面的基础上加一个排序规则就好了，就在插入的时候加一个判断

    import java.util.Stack;

    public class Deque<Item extends Comparable<Item>> {
        private final Stack<Item> a;
        private final Stack<Item> b;

        public Deque() {
            a = new Stack<Item>();
            b = new Stack<Item>();
        }

        public void push(Item t) {
            while (!a.empty() && a.peek().compareTo(t) < 0) {
                b.push(a.pop());
            }
            a.push(t);
            while (!b.empty()) {
                a.push(b.pop());
            }
        }

        public Item poll() {
            return a.pop();
        }
    }