946.Validate-Stack-Sequences.md

946. Validate Stack Sequences

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题目地址

https://leetcode.com/problems/validate-stack-sequences/

题目描述

Given two sequences pushed and popped with distinct values, return true if and only if this could have been the result of a sequence of push and pop operations on an initially empty stack.

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.
 
Note:
0 <= pushed.length == popped.length <= 1000
0 <= pushed[i], popped[i] < 1000
pushed is a permutation of popped.
pushed and popped have distinct values.

代码

Approach #1 Greedy

Time: O(N) && Space: O(N)

class Solution {
  public boolean validateStackSequences(int[] pushed, int[] popped) {
		int N = pushed.length;
    Stack<Integer> stack = new Stack();
    
    int j = 0;
    for (int x: pushed) {
      stack.push(x);
      while (!stack.isEmpty() && j < N && stack.peek() == popped[j]) {
        stack.pop();
        j++;
      }
    }
    
    return j == N;
  }
}