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# Validate Stack Sequences

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:**

1. `0 <= pushed.length == popped.length <= 1000`
2. `0 <= pushed[i], popped[i] < 1000`
3. `pushed` is a permutation of `popped`.
4. `pushed` and `popped` have distinct values.

```java
class Solution {
    public boolean validateStackSequences(int[] pushed, int[] popped) {
        int i = 0, j = 0;
        Deque<Integer> st = new LinkedList<>();
        while (i < pushed.length || j < popped.length) {
            // Pop if you can
            if (st.size() != 0 && st.peek() == popped[j]) {
                st.pop();
                j++;
            }
            // Else try to push
            else if (i < pushed.length) {
                st.push(pushed[i]);
                i++;
            } else
                break;
        }
        return st.size() == 0;
    }
}
```


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