946. Validate Stack Sequences

Medium

Given two integer arrays pushed and popped each with distinct values, return true if this could have been the result of a sequence of push and pop operations on an initially empty stack, or false otherwise.

Example 1:

Input: pushed = [1,2,3,4,5], popped = [4,5,3,2,1]
Output: true
Explanation: We might do the following sequence:
push(1), push(2), push(3), push(4),
pop() -> 4,
push(5),
pop() -> 5, pop() -> 3, pop() -> 2, pop() -> 1

Example 2:

Input: pushed = [1,2,3,4,5], popped = [4,3,5,1,2]
Output: false
Explanation: 1 cannot be popped before 2.

Constraints:

• 1 <= pushed.length <= 1000

• 0 <= pushed[i] <= 1000

• All the elements of pushed are unique.

• popped.length == pushed.length

• popped is a permutation of pushed.

解題

func validateStackSequences(pushed []int, popped []int) bool {
    stack := make([]int, 0)
    i := 0 

    for _, n := range pushed {
        stack = append(stack, n) // 先把值 push 進去
        
        // 檢查 stack 最後一個是不是目前要 pop 的
        // 可以的話就持續 pop
        for len(stack) != 0 && stack[len(stack)-1] == popped[i] {
            stack = stack[:len(stack)-1]
            i++ 
        }
    }

    return len(stack) == 0  // stack 被 pop 完了，代表可行
}