1441. Build an Array With Stack Operations

Given an array target and an integer n. In each iteration, you will read a number from  list = {1,2,3..., n}.

Build the target array using the following operations:

• Push: Read a new element from the beginning list, and push it in the array.
• Pop: delete the last element of the array.
• If the target array is already built, stop reading more elements.

You are guaranteed that the target array is strictly increasing, only containing numbers between 1 to n inclusive.

Return the operations to build the target array.

You are guaranteed that the answer is unique.

Example 1:

Input: target = [1,3], n = 3
Output: ["Push","Push","Pop","Push"]
Explanation: 
Read number 1 and automatically push in the array -> [1]
Read number 2 and automatically push in the array then Pop it -> [1]
Read number 3 and automatically push in the array -> [1,3]

Example 2:

Input: target = [1,2,3], n = 3
Output: ["Push","Push","Push"]

Example 3:

Input: target = [1,2], n = 4
Output: ["Push","Push"]
Explanation: You only need to read the first 2 numbers and stop.

Example 4:

Input: target = [2,3,4], n = 4
Output: ["Push","Pop","Push","Push","Push"]

class Solution {
    public List<String> buildArray(int[] target, int n) {
        List<String> res = new ArrayList();
        List<Integer> list = new ArrayList();
        for(int i: target) list.add(i);
        for(int i = 1; i <= n; i++){
            if(i > target[target.length - 1]) return res;
            if(list.indexOf(i) < 0){
                res.add("Push");
                res.add("Pop");
            }
            else {
                res.add("Push");
            }
        }
        return res;
    }
}

多种方法，比如把target存到arraylist里面，然后遍历从1-n，当n比target最大的还大，就没有必要继续了，返回即可

内循环中，如果target里面妹有n，就push再pop，否则push