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.
Constraints:
0 <= pushed.length == popped.length <= 1000
0 <= pushed[i], popped[i] < 1000
-
pushed
is a permutation ofpopped
. -
pushed
andpopped
have distinct values.
class Solution { public boolean validateStackSequences(int[] pushed, int[] popped) { int n = pushed.length; int j = 0; Stack<Integer> stack = new Stack(); for(int num : pushed) { stack.push(num); while(!stack.isEmpty() && stack.peek() == popped[j]) { stack.pop(); j++; } } return j == n; } }
greedy, 当栈顶等于popped时pop出,同时j++,最后如果所有数字都match上了就说明是对的。