leetcode 946. Validate Stack Sequences（有效栈序列）

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.

给两个数组，一个是要压栈的数字，一个是要出栈的数字。问经过这些出栈操作之后栈是否能恢复空。
即push, pop操作是否有效。

思路：
可用两个指针顺序指向pushed, popped中的数字，两个一样时直接跳过（不用先push再pop一遍），
不一样时先看stack中有没有可以pop的元素（即栈顶元素是否与pop元素相等），如果有，pop，然后pop指针右移；
如果没有, push入stack中，push指针右移（但是如果push已经到最右端，就返回挨个找栈顶是否有匹配元素，如果和栈顶不匹配，直接返回false)

//3ms
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int n = pushed.length;
        Stack<Integer> stack = new Stack<>();
        int push = 0;
        int pop = 0;
        
        while(pop < n) {
            if(push >= n) {
                if(stack.isEmpty() || stack.peek() != popped[pop]) return false;
                else stack.pop();
                pop ++;
            } else {
                if(pushed[push] != popped[pop]) {
                    if(!stack.isEmpty() && stack.peek() == popped[pop]) {
                        stack.pop();
                        pop ++;
                    } else {
                       stack.push(pushed[push]);
                       push ++; 
                    }
                
                } else {
                    push ++;
                    pop ++;
               
                }
                
            }
            
            
        }
        return true;
    }

或者是每次都push, 然后先看stack中有没有可以pop的，一下pop完，再进入下一个push。

//4ms
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int n = pushed.length;
        Stack<Integer> stack = new Stack<>();
        int pop = 0;
        
        for(int num : pushed) {
            stack.push(num);
            while(!stack.isEmpty() && stack.peek() == popped[pop]) {
                stack.pop();
                pop ++;
            }
        }
        return stack.isEmpty();
    }

数组模拟stack

//0ms
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int n = pushed.length;
        int[] st = new int[n];
        int head = -1; //栈顶
        int pushP = 0;  //pushed pointer
        int popP = 0;   //popped pointer

        for(int pushNum : pushed) {
            st[++head] = pushed[pushP++];
            while(head >= 0 && st[head] == popped[popP]) {
                head --;
                popP ++;
            }
        }
        return (head == -1);
    }