341. Flatten Nested List Iterator展開多層數組

［抄題］：

Given a nested list of integers, implement an iterator to flatten it.

Each element is either an integer, or a list -- whose elements may also be integers or other lists.

Example 1:
Given the list [[1,1],2,[1,1]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].

 

Example 2:
Given the list [1,[4,[6]]],

By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].

[暴力解法]：

時間分析：

空間分析：

 [優化後]：

時間分析：

空間分析：

［奇葩輸出條件］：

［奇葩corner case］：

［思惟問題］：

不知道.next 和 .hasnext有啥區別：取出來、只是看看有沒有

［一句話思路］：

只有stack才能一次取出來一層，getlist getinteger

［輸入量］：空： 正常狀況：特大：特小：程序裏處理到的特殊狀況：異常狀況（不合法不合理的輸入）：

［畫圖］：

［一刷］：

1. .next創建在.hasnext的基礎之上，因此hasnext須要放在while循環中，作完爲止
2. curr若是是個list，就必須用專有方法

   curr.getList()

   先取出，再作後續操做

［二刷］：

［三刷］：

［四刷］：

［五刷］：

  [五分鐘肉眼debug的結果]：

［總結］：

一次放一層

［複雜度］：Time complexity: O(n) Space complexity: O(n)

［英文數據結構或算法，爲何不用別的數據結構或算法］：

list 對應的方法是.size() .get

只有stack才能一次取出來一層，數組不能直接取出來一層。因此用stack。

stack有

.getInteger()

.getList()

方法

［算法思想：遞歸/分治/貪心］：

［關鍵模板化代碼］：

public NestedIterator(List<NestedInteger> nestedList) {
    //put into stack from back
        for (int i = nestedList.size() - 1; i >= 0; i--) stack.push(nestedList.get(i));
    }

［其餘解法］：

［Follow Up］：

［LC給出的題目變變變］：

 [代碼風格] ：

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * public interface NestedInteger {
 *
 *     // @return true if this NestedInteger holds a single integer, rather than a nested list.
 *     public boolean isInteger();
 *
 *     // @return the single integer that this NestedInteger holds, if it holds a single integer
 *     // Return null if this NestedInteger holds a nested list
 *     public Integer getInteger();
 *
 *     // @return the nested list that this NestedInteger holds, if it holds a nested list
 *     // Return null if this NestedInteger holds a single integer
 *     public List<NestedInteger> getList();
 * }
 */
public class NestedIterator implements Iterator<Integer> {
    //ini:stack
    Stack<NestedInteger> stack = new Stack<>();
    
    public NestedIterator(List<NestedInteger> nestedList) {
    //put into stack from back
        for (int i = nestedList.size() - 1; i >= 0; i--) stack.push(nestedList.get(i));
    }

    @Override
    public Integer next() {
    //pop
        return stack.pop().getInteger();
    }

    @Override
    public boolean hasNext() {
        while (!stack.isEmpty()) {
        //isInteger or put into stack from back
        NestedInteger curr = stack.peek();
        if (curr.isInteger()) return true;
        
        stack.pop();
        for (int i = curr.getList().size() - 1; i >= 0; i--) {
            stack.push(curr.getList().get(i));
        }
        
        
        }
        return false;
    }
}

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i = new NestedIterator(nestedList);
 * while (i.hasNext()) v[f()] = i.next();
 */

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