<p>You are given an array <code>books</code> where <code>books[i] = [thickness<sub>i</sub>, height<sub>i</sub>]</code> indicates the thickness and height of the <code>i<sup>th</sup></code> book. You are also given an integer <code>shelfWidth</code>.</p>

<p>We want to place these books in order onto bookcase shelves that have a total width <code>shelfWidth</code>.</p>

<p>We choose some of the books to place on this shelf such that the sum of their thickness is less than or equal to <code>shelfWidth</code>, then build another level of the shelf of the bookcase so that the total height of the bookcase has increased by the maximum height of the books we just put down. We repeat this process until there are no more books to place.</p>

<p>Note that at each step of the above process, the order of the books we place is the same order as the given sequence of books.</p>

<ul>
	<li>For example, if we have an ordered list of <code>5</code> books, we might place the first and second book onto the first shelf, the third book on the second shelf, and the fourth and fifth book on the last shelf.</li>
</ul>

<p>Return <em>the minimum possible height that the total bookshelf can be after placing shelves in this manner</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/06/24/shelves.png" style="height: 500px; width: 337px;" />
<pre>
<strong>Input:</strong> books = [[1,1],[2,3],[2,3],[1,1],[1,1],[1,1],[1,2]], shelfWidth = 4
<strong>Output:</strong> 6
<strong>Explanation:</strong>
The sum of the heights of the 3 shelves is 1 + 3 + 2 = 6.
Notice that book number 2 does not have to be on the first shelf.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> books = [[1,3],[2,4],[3,2]], shelfWidth = 6
<strong>Output:</strong> 4
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= books.length &lt;= 1000</code></li>
	<li><code>1 &lt;= thickness<sub>i</sub> &lt;= shelfWidth &lt;= 1000</code></li>
	<li><code>1 &lt;= height<sub>i</sub> &lt;= 1000</code></li>
</ul>

class Solution {
private:
    int n;
    int t[1001][1001]={-1}; 
    // use simple concept of keeping book at same shelf or next shelf 
    // recursive approach so using mwmoization works well here
    int solve(vector<vector<int>>& books,int rem_width,int shelfWidth,int i,int max_height){
        // keep in same shelf
        if(i>=books.size())
            return max_height;
        if(t[i][rem_width])
            return t[i][rem_width];
        int book_w=books[i][0];
        int book_h=books[i][1];
        int keep=INT_MAX; // if keep on same
        if(book_w<=rem_width){
            keep=solve(books,rem_width-book_w,shelfWidth,i+1,max(max_height,book_h));
        }
        // keep on next shelf
        int skip=max_height+solve(books,shelfWidth-book_w,shelfWidth,i+1,book_h);
        return t[i][rem_width]=min(keep,skip);
    }
public:
    int minHeightShelves(vector<vector<int>>& books, int shelfWidth) {
        // order of placing book should be same as given
        // greedy algorithm will not work here since we want to minimize the height
        // int prev_level=0;  // shelf ke prev level tak height
        // int ans=0;  // shelf ke current level tak height
        // int space_left=shelfWidth;
        // int n=books.size();
        // for(int i=0;i<n;i++){
        //     if((space_left-books[i][0])>=0){
        //         ans=max(ans,(prev_level+books[i][1])); // update the height
        //         space_left-=books[i][0];
        //     }
        //     // when current level does not have enough thickness
        //     else{
        //         space_left=shelfWidth-books[i][0];
        //         prev_level=ans;
        //         ans+=books[i][1];
        //     }
        // }
        // return ans;
        // now lets try out using the concept of dp
        // then for each book we have to decide whether to keep it in same shelf or next height
        // try both recursively and return min of them
        int rem_width=shelfWidth;
        int ans=solve(books,rem_width,shelfWidth,0,0);
        return ans;
    }
};
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