530. 二叉搜索树的最小绝对差

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方案一: 中序遍历

var getMinimumDifference = function(root) {
    let ans = Number.MAX_SAFE_INTEGER, pre = -1;
    const dfs = (root) => {
        if (root === null) {
            return;
        }
        dfs(root.left);
        if (pre == -1) {
            pre = root.val;
        } else {
            ans = Math.min(ans, root.val - pre);
            pre = root.val;
        }
        dfs(root.right);
    }
    dfs(root);
    return ans;
};