p494

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LeetCode p494 Target Sum 题解

1.题目：

You are given a list of non-negative integers, a1, a2, …, an, and a target, S. Now you have 2 symbols + and -. For each integer, you should choose one from + and - as its new symbol.

Find out how many ways to assign symbols to make sum of integers equal to target S.

Example 1:
Input: nums is [1, 1, 1, 1, 1], S is 3.
Output: 5
Explanation:

-1+1+1+1+1 = 3
+1-1+1+1+1 = 3
+1+1-1+1+1 = 3
+1+1+1-1+1 = 3
+1+1+1+1-1 = 3

There are 5 ways to assign symbols to make the sum of nums be target 3.
Note:
The length of the given array is positive and will not exceed 20.
The sum of elements in the given array will not exceed 1000.
Your output answer is guaranteed to be fitted in a 32-bit integer.

题意：

输入一个数组和一个目标数，数组中每个数前面可以填上正号或者负号，问有多少种方式，使数组的和为目标数。

2.解题思路:

dp 见代码

3.代码

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public class Solution {
    public int ans = 0;

	public int findTargetSumWays(int[] nums, int S) {

		dp(nums, 0, 0, S);
		return ans;
	}

	public void dp(int nums[], int i, int sum, int S) {
		if (i == nums.length) {
			if (sum == S)
				ans++;
			return;
		}
		dp(nums, i + 1, sum + nums[i], S);
		dp(nums, i + 1, sum - nums[i], S);
	}
}

4.一些总结：