题目： 

 

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解析： 

决策树： 

 

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代码设计： 

 

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代码： 

写法一：path为全局变量

private int ret,path,aim;
    public int findTargetSumWays(int[] nums, int target) {
        aim = target;
        dfs(nums,0);
        return ret;
    }
    
    private void dfs(int[] nums, int pos){
        if(pos == nums.length){
            if(path == aim) ret++;
            return;
        } 

        //添加 '+'
        path += nums[pos];
        dfs(nums,pos+1);
        path -= nums[pos];//回溯

        //添加 '-'
        path -= nums[pos];
        dfs(nums,pos+1); 
        path += nums[pos];//回溯
    }

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写法二：path作为参数

private int ret,aim;
    public int findTargetSumWays(int[] nums, int target) {
        aim = target;
        dfs(nums,0,0);
        return ret;
    }
    
    private void dfs(int[] nums, int pos,int path){
        if(pos == nums.length){
            if(path == aim) ret++;
            return;
        }

        //添加 '+'
        dfs(nums, pos+1, path + nums[pos]);

        //添加 '-'
        dfs(nums, pos+1, path - nums[pos]); 
    }