Backpack

思路：
State: dp[i][j]表示前i个石头，能不能恰好装满值j。
Transition: dp[i][j]=dp[i-1][j-A[i-1]] || dp[i-1][j]。这里的思路和house robbery很想，分2种状态，取或者不取i。取i要满足条件，那么前i-1个石头必须填满j-A[i-1]的值。不取i，那么前i-1个石头能够把j填满。

实现：
要检查j-A[i-1]是否大于0，否则会indexoutofbond.

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