Problem
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 ofpushed
.
Solution (Java)
class Solution {
public boolean validateStackSequences(int[] pushed, int[] popped) {
Deque<Integer> stack = new ArrayDeque<>();
int i = 0;
int j = 0;
int len = pushed.length;
while (i < len) {
if (pushed[i] == popped[j]) {
i++;
j++;
} else if (!stack.isEmpty() && stack.peek() == popped[j]) {
stack.pop();
j++;
} else {
stack.push(pushed[i++]);
}
}
while (j < len) {
if (!stack.isEmpty() && stack.peek() != popped[j++]) {
return false;
} else {
stack.pop();
}
}
return true;
}
}
Explain:
nope.
Complexity:
- Time complexity : O(n).
- Space complexity : O(n).